docs+test #176/#177: 12 mechanisms refuted, apparatus shipped, probe protocol staged

The dungeon render pair (purple seam flash + stair pop-in) resisted
desk root-causing, but the investigation narrowed the space to two
live theories and shipped permanent apparatus:

- Issue176177DungeonSeamInspectionTests: dat truth — corridor floors
  ARE textured drawn PortalSide portal polys; the reciprocal is NoPos;
  the 'stairs' are 0x8A020182's ramp shell (vertical portals, zero
  statics); CellBSP partitions exactly at portal planes; DXT1 textures
  carry zero transparent-mode texels.
- Issue176177FacilityHubFloodReplayTests: production-matched flood
  replays — approach/descent/gaze-sweep/walk all healthy with coherent
  inputs; ScenarioE pins the flood's collapse-to-root sensitivity under
  incoherent (root, eye) pairs.
- Issue176177SeamTransitLagTests: the resolver flips cells within one
  tick-step of the portal plane — the 0.33-0.47m [cell-transit] 'lag'
  in the gate logs is speed*tick quantization, not membership error.

Refuted (do NOT retry — ledger in the research doc): placeholder
texture, reciprocal z-fight, seal z-fight (seals only fire for
OtherCellId==0xFFFF), root/eye incoherence (production camera sweep is
mm-exact at planes), flood bistability, #119-class statics, undefined
DXT mips (both paths decode DXT->RGBA8; the compressed-array branch is
dead code), DXT1 alpha, fog mix (ramp ~538m), lightning leak (flash==0
in production), viewer-light pops (smooth (1-d/range) ramp).

Filed #178 (A8 double-sided shell stopgap still live) and #179
(lightning flash lacks an indoor gate — dormant). The purple can only
be the fog clear color (undrawn pixels) or the outdoor ambient+sun
tint; discrimination needs ONE probe launch (ACDREAM_PROBE_LIGHT +
ACDREAM_PROBE_PVINPUT + ACDREAM_PROBE_CELL) — protocol in
docs/research/2026-07-06-176-177-render-pair-investigation.md.

Suites: Core 2587 / App 719 green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/research/2026-07-06-176-177-render-pair-investigation.md

@@ -0,0 +1,134 @@
+# #176/#177 render pair — investigation ledger (2026-07-06, session 2)
+
+**Status: mechanism NOT yet pinned — but the hypothesis space is now razor-thin.**
+Twelve candidate mechanisms refuted by direct evidence (dat dumps, headless
+replays, production-log analysis, code reads). Every layer that can be checked
+offline is verified HEALTHY at the anchor cells. The surviving discriminator
+requires ONE live probe launch (protocol at the bottom — piggyback on the
+pending #175 door gate).
+
+## The issues
+
+- **#176** — purple flashing on dungeon floors at cell seams, camera-angle
+  dependent (Facility Hub).
+- **#177** — stairs pop in/out across levels: (a) vanish on the last step
+  running down, (b) invisible looking into the stair room from the corridor,
+  (c) pop into existence on entering.
+
+## Anchor-cell dat truth (Issue176177DungeonSeamInspectionTests)
+
+- Corridor `0x8A02016E` floor = three abutting TEXTURED drawn portal polys
+  (polys 1/3/5, surface 0x08000377 → DXT1 tex 0x050026F7, `[PortalSide]`) →
+  under-hall `0x011E` (z=−12). Reciprocal ceiling poly = NoPos (not drawn).
+- The "stairs" = a RAMP owned by `0x8A020182` (inclined drawn polys, z −9…−6
+  floor + −6…−3 ceiling); `0x0183` = flat lower cell. Connections are
+  VERTICAL wall portals (NOT floor-portals). PortalSide flag asymmetry:
+  0x0182→0x0183 carries PortalSide; the back-portal does not.
+- **Zero StaticObjects in all five anchor cells** (no #119-class statics, no
+  torch-bearing stabs → no registered point lights except the viewer fill).
+- CellBSP volumes partition EXACTLY at the portal planes (no overlap zone).
+- All surfaces resolve; DXT1 textures contain **zero** transparent-mode
+  texels (all 3-color-mode blocks, index 3 never used).
+
+## REFUTED mechanisms (each by direct evidence — do NOT retry)
+
+| # | Hypothesis | Killed by |
+|---|---|---|
+| 1 | Placeholder/missing texture (magenta class) | All surfaces resolve; drawn-poly sweep 0 misses |
+| 2 | Reciprocal portal-poly z-fight | Reciprocal is NoPos (never drawn) |
+| 3 | Seal depth-stamp z-fights the drawn floor-portal | Seals fire ONLY for `OtherCellId==0xFFFF` (GameWindow:11437); a sealed dungeon draws zero seals |
+| 4 | Root/eye incoherence (viewer root lags the eye across portal planes) | Production camera sweep publishes coherent pairs; out-cell flips at x=85.001/88.335 — mm-exact at the planes (gate2 log). The flood DOES collapse to 1 cell under artificially incoherent inputs (ScenarioE pin) — but production inputs are coherent |
+| 5 | Membership transit lag (0.33–0.47 m in [cell-transit]) as the render-root lag | The resolver flips within one tick-step of the plane in the harness (Issue176177SeamTransitLagTests); the logged "lag" is speed×tick quantization of the PLAYER probe, and the camera root (probe out-cell) is plane-exact |
+| 6 | Flood bistability at the anchors | ScenarioC gaze sweep (2° steps, 4 pitches): 0 one-step drops; ScenarioA stair approach: ramp+lower admitted at all tested eyes/pitches |
+| 7 | Staircase = EnvCell static culled by viewcone (#119 class) | Zero statics in the anchor cells; the stairs are shell geometry |
+| 8 | Undefined DXT mip levels (compressed arrays skip GenerateMipmap) | Both ObjectMeshManager texture paths DECODE DXT→RGBA8 (BcDecoder) — the compressed-array branch of ManagedGLTextureArray is dead WB-heritage code; RGBA8 arrays get real mips |
+| 9 | DXT1 3-color-mode alpha=0 texels + opaque-pass `discard a<0.05` / A2C | Block histogram: 0 transparent texels in all Hub floor/wall textures |
+| 10 | Fog mix toward purple FogColor at distance | Fog ramp starts at `_nearRadius×192×0.7 ≈ 538 m` (radii stay 4/12 in dungeon mode); Hub sightlines ≤ ~100 m → fog term ≡ 0 |
+| 11 | Lightning-flash additive (`uFogParams.z × (0.6,0.6,0.75)`) leaking indoors | `WeatherState._flashLevel` is 0 in production ("Production never TriggerFlashes") — dormant test hook. (The missing indoor gate is still real debt for when storm strobes ship.) |
+| 12 | Viewer-light per-cell/per-vertex pops (hard range edge or 8-set membership flips) | The point ramp is `(1−d/range)` — smoothly ZERO at the range boundary; set-membership beyond range is a zero-contribution no-op. No torches exist in the Hub cells to churn the 8-cap |
+
+## VERIFIED-HEALTHY layers (offline pins, keep as regression assets)
+
+- `PortalVisibilityBuilder` at the anchors: approach/descent/gaze-sweep/walk
+  scenarios (`Issue176177FacilityHubFloodReplayTests`) — admissions correct
+  and stable with coherent inputs; the ScenarioE incoherent-input collapse is
+  the sensitivity pin (1-cell flood when root≠eye side).
+- Membership: `ResolveWithTransition` flips cells within one tick of the
+  portal plane, both directions (`Issue176177SeamTransitLagTests`).
+- Mesh path: `CellMesh.Build` (production, GameWindow:7013) draws the
+  textured floor-portal strips; snapshot frustum gate (WbFrustum) is the
+  standard conservative p-vertex test; per-cell AABBs are vertex-derived,
+  8-corner transformed.
+- Per-instance light sets are truly per-instance (EnvCellRenderer MDI).
+
+## SURVIVING theories (need the live discriminator)
+
+- **T-A (ambient flip):** any frame where `playerRoot` resolves null (or
+  `playerSeenOutside` defaults true) runs the OUTDOOR lighting branch —
+  purple sky ambient + full sun. Sun is directional-from-above → floors
+  (N·L≈1) catch it, walls (N·L≈0) barely → a FLOOR-selective purple-bright
+  flash, temporally lockable to whatever gaps CurrCell/TryGetCell. Desk
+  analysis found no per-frame gap trigger during plain corridor runs
+  (UpdatePlayerCurrCell is stale-beats-null; the registry is stable in
+  AP-36 dungeon mode) — but the branch inputs are exactly probed by
+  `[light] insideCell=` lines, so one run settles it.
+- **T-B (flood with REAL production inputs):** my harness feeds synthetic
+  viewProj/eye. If the real per-frame inputs at the artifact moments differ
+  (collided camera pressed into walls near seams, damped-forward gaze), the
+  flood could still misbehave in configurations the sweeps missed.
+  `[pv-input]` prints the exact inputs + flood count per frame.
+- **T-C:** unknown-unknown (GPU state, driver, MSAA resolve…). If T-A/T-B
+  both read clean at a flash moment → RenderDoc frame capture next.
+
+## Also found (real, filed, not these bugs)
+
+- **A8 double-sided shells stopgap still live**: `EnvCellRenderer.cs`
+  RenderModernMDIInternal maps `CullMode.Landblock → CullMode.None`
+  ("while the architectural cause is isolated") — all cell shells draw
+  two-sided. Perf + correctness debt; retire under the A7/A8 lighting arc.
+- **Lightning indoor gate missing** (dormant): when weather strobes ship,
+  `SceneLightingUbo.Build` needs the `playerInsideCell` gate or dungeons
+  will strobe blue-violet.
+- **Flood follows floor-portals DOWNWARD from above** (ScenarioC: under-hall
+  network admitted at down-pitches). Retail's `portal_side` side test
+  (0x005a59a0: portal_side≠0 → NEGATIVE side only; ==0 → POSITIVE only;
+  IN_PLANE(±0.0002) → refused) *appears* to refuse this direction, but my
+  plane-sign reading had unresolved contradictions (CCellPortal::UnPack
+  normalizes portal_side at load, 0x0053ba1c/0x0053bc6a). OPEN QUESTION —
+  harmless-looking (extra admitted cells draw below the opaque floor,
+  z-buffer wins) but worth settling when the flood is next touched.
+
+## The live probe protocol (piggyback on the #175 door gate launch)
+
+Env (add to the standard launch block):
+
+```
+ACDREAM_PROBE_LIGHT=1      # [light] insideCell/ambient/sun — rate-limited
+ACDREAM_PROBE_PVINPUT=1    # [pv-input] one line/frame: exact flood inputs + count
+ACDREAM_PROBE_CELL=1       # [cell-transit] timeline anchors
+```
+
+User reproduces in Facility Hub (~2 min): run the corridor across several
+seams until the purple flash shows; approach the stair room from the
+corridor, walk in, run down. Then close. Discrimination:
+
+- Flash moments + `[light]` shows `insideCell=False` blips or ambient jumps
+  → **T-A confirmed** (then root-cause the gap trigger from the same log).
+- `[pv-input]` flood count drops (e.g. 12→1) at flash/pop moments while
+  `[light]` stays clean → **T-B confirmed** (the line carries the exact
+  inputs to replay in `Issue176177FacilityHubFloodReplayTests`).
+- Stairs invisible while the flood contains 0x0182/0x0183 → draw-side hunt
+  (RenderDoc next); absent → flood-side with the printed inputs.
+- All clean → T-C: RenderDoc frame capture of a flash frame.
+
+## Apparatus shipped this session
+
+- `tests/AcDream.Core.Tests/Rendering/Issue176177DungeonSeamInspectionTests.cs`
+  — dat truth: portal polys/surfaces/draw verdicts, reciprocal coincidence,
+  stair geometry owner, CellBSP containment, under-hall surface colors,
+  DXT1 alpha histograms.
+- `tests/AcDream.App.Tests/Rendering/Issue176177FacilityHubFloodReplayTests.cs`
+  — production-matched flood replays: stair approach, descent, gaze sweep,
+  corridor walk, and the ScenarioE incoherent-root sensitivity pin.
+- `tests/AcDream.Core.Tests/Physics/Issue176177SeamTransitLagTests.cs`
+  — resolver cell-flip position at the seam (plane-exact pin).