docs(research): Phase A8 RR0 — Issues A + C caused by R3, NOT pre-existing

Three-branch falsification spike per the design's RR0:

  HEAD (2bfeafd, R3.5 v2):     Issue C YES; Issue A YES (varies by building)
  R3 baseline (60f07bc):        Issue C YES; Issue A YES (same as HEAD)
  main (7034be9, no A8 work):   Issue C NO; Issue A NO flicker; BUT
                                 constant #78 symptom (houses-below-terrain
                                 visible from inside)

Diagnosis: R3 (stencil pipeline wire-in) successfully fixes the original
#78 main symptom but introduces Issues A and C as new transition artifacts.
R3.5 v1+v2 patches didn't help (R3 baseline shows same A+C as HEAD).

Per the design's decision gate (Outcome 2): PAUSE plan; re-brainstorm
via superpowers:brainstorming to address A+C without re-introducing #78
constant leak.

The original restructure design assumed A+C might be pre-existing and
could be filed as separate out-of-A8-scope issues. RR0 invalidates that.
The restructure must address them OR the brainstorm needs a third option
between "stencil-gate everything" (causes A+C) and "no stencil work"
(causes #78).

Open questions for the re-brainstorm captured in the findings doc.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/research/2026-05-26-a8-rr0-falsification-findings.md

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+# A8 RR0 falsification — are Issues A and C pre-existing or A8-caused?
+
+**Date:** 2026-05-26 (PM)
+**Method:** three-branch launch + visual repro at Holtburg cottage entry / exit transitions.
+
+## Observations
+
+| Branch | Commit | Issue C (entry transparent floor) | Issue A (exit through-ground / walls missing) | #78 (constant houses-below-terrain visible from inside) |
+|---|---|---|---|---|
+| HEAD | `2bfeafd` (R3.5 v2) | **YES** | **YES** (varies by building) | (gone — R1+R2+R3 fixed) |
+| R3 baseline | `60f07bc` | **YES** | **YES** (same as HEAD) | (gone — R1+R2+R3 fixed) |
+| main | `7034be9` | **NO** | **NO flicker** — but DIFFERENT SYMPTOM: houses below terrain visible from inside, constant (not transition) | **YES, constant** |
+
+User screenshots from HEAD captured during the spike:
+1. Cottage interior: floor partly see-through to outdoor grass; misplaced textured panel visible
+2. Cottage exterior: brown floor/wall panel floating in space; surrounding building walls missing
+
+User quote on main observation:
+> "No floor is not transparent."
+> "When I now stand in the cottage and look out I can see houses below the terrain. There is no flick when I pass out. They are just visible all the time"
+
+## Diagnosis
+
+Issues A and C are **NOT pre-existing.** They are caused by **R3 (the stencil pipeline wire-in)**:
+
+- R3 successfully closes the original #78 symptom (constant houses-below-terrain visibility from inside) ✓
+- R3 introduces two new artifacts as side-effects:
+  - Issue C — cottage floor transparent showing cellar during entry transition
+  - Issue A — through-ground objects + walls-missing flicker during exit transition
+
+The R3.5 v1+v2 patches were attempts to mitigate, didn't help (R3 baseline and HEAD show identical A+C symptoms).
+
+## Decision
+
+Per the design's decision gate at RR0-S5:
+
+- [x] **Outcome 2 selected:** Only R3 + HEAD reproduce → A and/or C caused by R3 work specifically. **PAUSE the plan.** Re-brainstorm via `superpowers:brainstorming` to address them; update the design doc; resume.
+
+The original restructure design assumed Issues A and C might be pre-existing and could be filed as separate out-of-A8-scope issues. RR0 invalidates that assumption. The restructure must address them OR accept that A8 trades one bug class for another (which the user has not approved).
+
+## Open questions for the re-brainstorm
+
+1. **Mechanism of Issue C (entry transparent floor):** what about R3's stencil work makes cottage floor transparent during entry? Hypotheses:
+   - Stencil bit 1 set on portal silhouettes but cleared next frame; during the entry the camera-cell hadn't yet promoted, so VisibleCellIds was empty, so MarkAndPunch had no portals to mark → outdoor pass effectively ungated, terrain re-draw beats indoor cell mesh at the floor pixels.
+   - Depth-clear-if-inside firing too early or too late, leaving the depth buffer in a bad state.
+   - The cottage cell's mesh + the cellar cell's mesh both included in IndoorPass at adjacent Z values, Z-fight is fundamental.
+
+2. **Mechanism of Issue A (exit through-ground flicker):** during grace frames after exit, `cameraInsideCell=true` but `cameraReallyInside=false`. Sky skipped, terrain drawn, depth-clear skipped, stencil branch skipped, outdoor Draw(All) runs. Why do entities below terrain win the depth test in these specific frames?
+
+3. **Will the WB-faithful restructure help, hurt, or be neutral on A and C?** The restructure removes the depth-clear and initial-terrain workarounds. During grace frames after exit, it gates terrain on `!cameraInside` (true since cameraInside is strict). So terrain DRAWS unconditionally during grace (because !cameraInside = !false = true → draws). Behavior identical to main during these frames → likely re-introduces #78 main symptom for ~3 grace frames after exit. Trade-off: 3 frames of #78 vs 3 frames of Issue A.
+
+4. **Should we shorten or eliminate the cell-switch grace mechanism?** Currently 3 frames. If 0 frames, the gate flips strict and cleanly at the threshold. PointInCell epsilon (0.01f) provides minimal hysteresis but might be enough.
+
+5. **Is there a third option** between "stencil pipeline gates outdoor visibility" (causes A+C) and "no stencil work" (causes #78)? Possibilities:
+   - Stencil work but with different cell-set scoping (only camera-cell's portals, not BFS-extended; already in R3).
+   - Hybrid: stencil-gate outdoor scenery but NOT terrain (let terrain draw unconditionally + accept #78 leak for terrain only).
+   - Frame-based heuristic: skip stencil for first N frames after entry/exit to mask the transition artifact.
+
+## Logs
+
+- `launch-a8-rr0-head.log` / `launch-a8-rr0-head-take2.log` / `launch-a8-rr0-head-take3.log` — HEAD launches (2bfeafd)
+- `launch-a8-rr0-r3.log` — R3 baseline launch (60f07bc, GameWindow.cs single-file checkout)
+- `launch-a8-rr0-main.log` — main launch (7034be9, side worktree at .claude/worktrees/tmp-main-baseline with WorldBuilder ref junction)
+
+## Cleanup performed
+
+- Restored HEAD's GameWindow.cs in this worktree (no working-tree changes left)
+- Removed Windows junction `tmp-main-baseline/references/WorldBuilder` → strange-albattani-3fc83c
+- Removed side worktree `.claude/worktrees/tmp-main-baseline`