fix(physics): enforce retail step-down support radius (#273)

docs/research/2026-07-31-issue273-tight-gap-support.md

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+# Issue #273 — Holtburg tight-gap support validation
+
+**Date:** 2026-07-31
+**Status:** implementation, automated gates, and exact live gate pass
+**Scope:** grounded player step-down support at a floor edge beside a static
+cylinder
+
+## Captured scene
+
+The reproducible gap is in outdoor cell `0xA9B40032`, between:
+
+- building shell GfxObj `0x01000F69`, placed at
+  `(158.178, 37.7055, 94.0)` with quaternion
+  `(w=.939319, x=0, y=0, z=-.343045)`;
+- static post `0xCA9B4027`, placed at `(160.173, 34.487, 95.975)`,
+  represented by its Setup-authored cylinder (`radius=.282`,
+  `height=5.564`);
+- the local player Setup's exact two spheres (`radius=.48`, origins
+  `z=.475` and `z=1.35`).
+
+The building's supporting ledge terminates at local `x=4`. The first
+post-side response moved the player's foot-sphere center to approximately
+local `x=4.33`. The full `.48` movement sphere still overlapped the floor, so
+the existing step-down path accepted the candidate. Repeated frames then
+carried the player around the post and outside the building shell.
+
+The fixture
+`tests/AcDream.Core.Tests/Fixtures/issue273/0x01000F69.gfxobj.json` preserves
+the installed DAT PhysicsBSP. The replay in
+`Issue273HoltburgTightGapReplayTests` uses the captured object placement,
+player spheres, static posts, and movement offsets.
+
+## Retail mechanism
+
+The missing rule is not extra collision padding and is not a larger player
+sphere. It is retail's second-stage support validation:
+
+1. `CTransition::step_down` (`0x0050B2A0`) performs the ordinary downward
+   collision probe.
+2. After finding a walkable contact plane, an EdgeSlide mover that is not in
+   StepUp calls `CTransition::check_walkable` (`0x0050AFF0`). The binary
+   sequence is `test ah,2` at `0x0050B36A`, which is state bit `0x200`
+   (`EdgeSlide`), followed by the `step_up == 0` test and call at
+   `0x0050B380`.
+3. `CTransition::check_walkable` first calls
+   `SPHEREPATH::check_walkables` (`0x0050C3E0`).
+4. `SPHEREPATH::check_walkables` halves the saved foot-sphere radius and
+   calls `CPolygon::check_walkable` (`0x00538E60`).
+5. If the remembered polygon does not support that smaller sphere,
+   `CTransition::check_walkable` performs a downward CheckWalkable insertion.
+   BSP leaves require both `walkable_hits_sphere` and
+   `CPolygon::check_small_walkable` (`BSPLEAF::hits_walkable`,
+   `0x0053D670`).
+6. If neither check finds support, `CTransition::step_down` rejects the
+   candidate and the existing edge-response chain handles it.
+
+ACDream already had the small-radius BSP-leaf test, but
+`DoCheckWalkable` treated the mere presence of a remembered polygon as
+success, and the ordinary `DoStepDown(..., runPlacement:false)` path never
+called it. This let a full-radius overlap stand in for actual foot support.
+
+## Port
+
+- `BSPQuery.CheckWalkableSupport` is the shared resolved-polygon form of
+  retail `CPolygon::check_walkable`.
+- `SpherePath.CheckWalkables` implements the retail half-radius remembered
+  polygon check without mutating canonical sphere state.
+- `Transition.DoCheckWalkable` now tests the remembered polygon rather than
+  treating a non-null polygon as sufficient.
+- `Transition.DoStepDown` restores the EdgeSlide/non-StepUp support gate
+  before the existing placement-policy seam.
+
+There are no location checks, object IDs, guessed radii, widened collision
+shapes, or gap-specific tolerances in the production fix.
+
+## Regression impact
+
+The existing #271 staircase-side replay begins with its center `.288 m`
+outside a tread whose retail half-radius support boundary is `.24 m`.
+Retail may therefore stop that exact candidate. The test now preserves the
+original user-visible invariant—never reverse or accelerate downhill—without
+requiring forward progress beyond retail's support boundary. The ordinary
+continuous staircase replay still requires and achieves forward progress.
+
+## Gates
+
+- issue #273 fixture/replay: 3 passed;
+- focused BSP, step-up, edge-slide, #185/#271 family: 42 passed / 1 skipped;
+- complete Core tests: 4,111 passed / 2 skipped;
+- Release solution build: passed;
+- complete Release solution tests: 10,068 passed / 5 skipped.
+
+The user accepted the exact in-client Holtburg gap gate on 2026-07-31: the
+gap blocks from the tested approach, and the adjacent movement checks remain
+healthy.