docs(milestones): promote indoor work to M1.5 — primary focus

Continued indoor testing through 2026-05-20 surfaced a deep family
of physics + lighting bugs that span buildings AND dungeons. Today's
session shipped 5 surgical fixes (A4 + #89 + #90 + #91 + #92) that
close the user-visible "walls walk through at Holtburg inn" symptom,
but #90 specifically is a CLAUDE.md-rules workaround (sphere-overlap
stickiness on top of point-only cell containment) added without prior
approval. The underlying issue (BSP push-back distance probably
diverges from retail) hasn't been measured. Plus the umbrella #83
(indoor multi-Z walking) has been open since 2026-05-19 with multiple
aborted fix attempts; plus indoor lighting (#80 + #81 + new #93 +
#94) has been deferred as "M7 polish" but is actually part of the
same indoor-experience problem.

Promoting to a milestone of its own forces the work to be central,
retail-anchored, and complete — not another whack-a-mole patch.

Milestone M1.5 — "Indoor world feels right":
  Demo: enter Holtburg Sewer through the in-town portal, navigate
  through 5-7 rooms with stairs + a multi-Z chamber, exit back to
  town. Walls block. Stairs work. Items block. Lighting reads
  correctly. Cell transitions smooth.

  Phases:
    A6 — Indoor physics fidelity (cdb-driven)
    A7 — Indoor lighting fidelity (RenderDoc + retail-decomp driven)

  Issues in scope: #80, #81, #83, #88, #90 (workaround removal),
  #93 (new lighting umbrella), #94 (held-item spotlight),
  + TryFindIndoorWalkablePlane synthesis removal.

M2 ("Kill a drudge") deferred until M1.5 lands.

This commit updates:
  - docs/plans/2026-05-12-milestones.md (M1.5 block inserted, M2 moved
    to deferred status)
  - docs/plans/2026-04-11-roadmap.md (A6 + A7 sub-pieces detailed)
  - CLAUDE.md (Currently working toward updated to M1.5, M2 paragraph
    marked deferred, M1.5 baseline shipped paragraph added)
  - docs/ISSUES.md (#80, #81, #83, #88, #90 tagged M1.5 scope;
    new #93 indoor lighting umbrella + #94 held-item spotlight filed)
  - docs/research/2026-05-21-open-items-pickup-prompt.md (landscape
    table reorganized around M1.5 phases)

A6 + A7 specs to be drafted in the next session(s).

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

docs/plans/2026-04-11-roadmap.md

@@ -85,6 +85,99 @@ Plus polish that doesn't get its own phase number:
 
 ## Phases ahead — agreed order
 
+### Milestone M1.5 — "Indoor world feels right" (active, opened 2026-05-20)
+
+The current top of the work order. Two phases (A6 + A7) inside one
+milestone. M2 ("kill a drudge") is deferred until M1.5 lands —
+drudges live in dungeons and the M2 demo target requires solid indoor
+navigation. Full milestone block in
+[`docs/plans/2026-05-12-milestones.md`](2026-05-12-milestones.md).
+
+**Today's pre-M1.5 baseline** (2026-05-20 — committed in this
+session): A4 multi-cell BSP iteration (`691493e`), #89 sphere-overlap
+in CheckBuildingTransit (`7ac8f54`), #90 sphere-overlap stickiness in
+ResolveCellId (`4ca3596` — **WORKAROUND**, scheduled for removal in
+A6.P4), #91 indoor cell shadows in FindObjCollisions (`c0d8405`),
+#92 server cell id at player-mode entry (`23ab173`). 1147 + 8 baseline
+maintained. Holtburg inn + cottage interiors visually verified
+2026-05-20.
+
+#### Phase A6 — Indoor physics fidelity (cdb-driven)
+
+**Hypothesis:** Our `BSPQuery.FindCollisions` 6-path dispatcher (and
+its callers) produce collision responses that systematically diverge
+from retail's. Symptoms in different geometry (doorways, stairs,
+multi-Z, cellars, dungeons) share one underlying mechanism — most
+likely push-back distance / direction / CP synthesis.
+
+**Investigation methodology:** cdb-attached comparison. Toolchain
+documented in CLAUDE.md's "Retail debugger toolchain" section. Used
+successfully 2026-04-30 for the steep-roof case. Matching binaries
+(acclient.exe v11.4186) + PDB present.
+
+**Sub-pieces (slices):**
+- **A6.P1 — cdb probe spike** (~3 days). Build cdb scripts capturing
+  retail's per-tick state at 9 scenarios:
+  - 4 building sites: Holtburg inn doorway, inn stairs, inn 2nd floor,
+    cottage cellar.
+  - 5 dungeon sites: Holtburg Sewer entry portal, first stair descent,
+    inter-room portal transition, open central area, dark corridor.
+  Breakpoints on `set_collide`, `step_sphere_up`, `step_sphere_down`,
+  `transitional_insert`, `set_contact_plane`, `validate_walkable`.
+  Mirror with our equivalent probes (`[indoor-bsp]`, `[cp-write]`,
+  new `[push-back]`).
+- **A6.P2 — Analysis report** (~1 day). Quantify the per-call-site
+  gap. Identify which BSP path(s) over- or under-correct. Output:
+  1–3 specific bug findings with retail decomp anchors.
+- **A6.P3 — Fix the BSP correction paths** (~3–5 days). Surgical
+  fixes informed by A6.P2 data. Likely touches `BSPQuery.AdjustSphereToPlane`,
+  `AdjustOffsetToPlane`, Path 5 / Path 6 branches, sub-step state
+  mutation.
+- **A6.P4 — Remove workarounds** (~1 day). Revert #90 sphere-overlap
+  stickiness in `PhysicsEngine.ResolveCellId`. Delete
+  `Transition.TryFindIndoorWalkablePlane` + its caller in
+  `FindEnvCollisions`. Verify behavior holds without the workarounds.
+
+#### Phase A7 — Indoor lighting fidelity (RenderDoc + retail-decomp driven)
+
+**Hypothesis layers (less mapped than physics):**
+- Per-cell environment-light tag association — indoor cells should
+  inherit only their own env lights, not outdoor day-cycle.
+- Light visibility culling — what lights actually contribute to each
+  cell's render.
+- Per-entity light direction transform — held-item-spotlight bug
+  (#L-spotlight) is per-entity attribution gone wrong.
+- Static-stab atmospheric inheritance (#81).
+
+**Investigation methodology:** less existing infrastructure than
+physics. Requires:
+- New `[indoor-light]` probe (per-frame dump of active lights for the
+  player's cell + each visible entity: position, color, attenuation,
+  direction).
+- RenderDoc frame capture at the same 9 scenarios as A6.
+- Grep retail's `Render::lighting_*` family in
+  `acclient_2013_pseudo_c.txt` to map per-cell light selection logic.
+
+**Sub-pieces (slices):**
+- **A7.L1 — Lighting probe spike** (~3–5 days). Build `[indoor-light]`
+  probe. Capture baselines at all 9 scenarios. RenderDoc captures
+  paired with each. Decomp study of retail's lighting selection.
+- **A7.L2 — Analysis report** (~1–2 days). Likely surfaces 2–4
+  distinct bugs across the lighting issues.
+- **A7.L3 — Fix lighting paths** (~3–7 days). Wide variance because
+  the surface area is unknown. Could touch indoor env-light parsing,
+  `LightingHookSink`, WB rendering pipeline, shader uniforms.
+
+**M1.5 acceptance criterion (shared by A6 + A7):** Walk Holtburg Sewer
+end-to-end. Walls block (physics). Stairs work (physics). Items
+block (physics). Lighting reads correctly throughout (lighting).
+Cell transitions are smooth (physics). No regressions in M1 outdoor
+behavior. Estimated 17–26 days focused work, 3–5 weeks calendar.
+
+**Specs:** to be written 2026-05-20 (after milestone commit lands).
+
+---
+
 ### Phase A — Foundation (in progress)
 
 **Goal:** walk across 10+ landblocks without crashes, without hitches at landblock boundaries, and without framerate cratering.