docs: #176/#177 handoff — root-caused, folded into A7 dungeon lighting

Session handoff for the next pickup. #176 (purple seam flash) + #177
(stair-room light pop-in) are ONE bug, root-caused to the camera-nearest
MaxGlobalLights=128 snapshot cap evicting in-range lights of visible
cells (Hub = 366 fixtures). Fix deferred to A7 because uncapping exposes
unported per-cell light-reach (through-floor light) + the fixture
falloff-curve misassignment + an unattributed striped-floor artifact.

- New handoff: docs/research/2026-07-06-176-177-handoff-A7-lighting.md
  (root cause, why-deferred, the A7 fix order, tooling inventory).
- Roadmap A7 updated: now owns #176/#177; the 'light visibility culling'
  hypothesis layer is CONFIRMED (no per-cell insert_light registration);
  fixture-curve layer added. Analysis pre-paid.

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

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

@@ -293,14 +293,32 @@ successfully 2026-04-30 for the steep-roof case. Matching binaries
 
 #### Phase A7 — Indoor lighting fidelity (RenderDoc + retail-decomp driven)
 
+**Now also owns #176/#177 (2026-07-06):** the Facility Hub purple seam
+flash + stair-room light pop-in are ROOT-CAUSED to this phase's "light
+visibility culling" layer — a camera-nearest `MaxGlobalLights=128`
+snapshot cap evicts in-range lights of visible cells (Hub has 366
+fixtures), so per-cell 8-light sets churn as the camera moves.
+Uncapping was live-tested and reverted because the full pool exposes the
+per-cell-reach + fixture-curve defects below (through-floor light,
+1/d-vs-1/d³). Analysis PRE-PAID — see
+`docs/research/2026-07-06-176-177-handoff-A7-lighting.md` (the fix order:
+per-cell `insert_light` registration → static fixture curve → stripe
+hunt → uncap) + register AP-85.
+
 **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.
+  cell's render. **CONFIRMED bug here (#176/#177): no per-cell light
+  registration — lights are a flat world-space sphere-overlap pool that
+  reaches through solid floors; retail's `insert_light` 0x0054d1b0
+  scopes each light to its cell.**
 - Per-entity light direction transform — held-item-spotlight bug
   (#L-spotlight) is per-entity attribution gone wrong.
 - Static-stab atmospheric inheritance (#81).
+- **Fixture falloff curve (#176/#177): stationary server-spawned
+  fixtures ride the DYNAMIC 1/d path (#143 `isDynamic`); should be the
+  static 1/d³ bake (`calc_point_light` 0x0059c8b0).**
 
 **Investigation methodology:** less existing infrastructure than
 physics. Requires: