BT410 5.3.101: MECH4 pre-TU notes -- the ProcessCollision decode, two donor hazards that do not apply, and the re-hosting ledger

Captures the @004abb40 decode (base head + Mover-family branch with the
separating gate and mech-vs-mech ram + CulturalIcon branch with the 0x8000
stopping flag and the 0.00123f walk-through sentinel + the material tail)
before implementation, so the analysis survives session boundaries.

Also records two hazards BT411 fought that do NOT apply to this engine --
the compounded-reflection economy guard (our ProcessCollisionList restores
velocity per contact, MOVER.CPP:1203) and the AlarmIndicator ODR split --
plus the ledger of master-perf pieces currently re-hosted in Mech::Simulate
at STAGED seams, for the day mech4.cpp exists and they move home.

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

restoration/source410/BT/MECH4.NOTES.md

@@ -0,0 +1,85 @@
+# MECH4 — pre-TU reconstruction notes
+
+**Status: no MECH4.CPP exists yet.** The ground snap, frame rejection,
+knockdown stagger and collision economy that the binary keeps in mech4's
+master performance are currently RE-HOSTED inside `Mech::Simulate`
+([[MECH]], 5.3.98–100) at seams marked STAGED. This file carries the decode
+work for the pieces not yet written, so it survives session boundaries.
+
+## Mech::ProcessCollision (@004abb40) — decoded, not yet reconstructed
+
+The override of the engine's protected virtual (`MOVER.CPP:1354`), called
+per contact by `Mover::ProcessCollisionList`. The decomp
+(part_012.c:15206–15416) reads as the BASE function plus four additions:
+
+1. **The base head, verbatim**: `collisionVolume->ProcessCollision(collision,
+   worldLinearVelocity, lastCollisionList, &damage->surfaceNormal,
+   &penetration)`; on a miss NOTHING runs. Clamp penetration to the slice,
+   ratio = pen/dt, `damage->damageAmount = StaticBounce(old, dt, r, normal,
+   &elasticity, minimumBounceSpeed, &friction)` with the elasticity/friction
+   locals seeded from the mover's coefficients.
+
+2. **The Mover-family branch** (owner = the collided solid's owner entity,
+   solid+0x1c; family test FUN_0041a1a4 against the "Mover" derivation GUID
+   @0x4e4518):
+   * SEPARATING-CONTACT GATE: `(ourWorldVel − other->WorldVel[vtbl+0x20])
+     · normal < −1e-4` (`_DAT_004ac044`) → return, no damage, no dispatch.
+   * If the other's classID == **0xbb9** (mech), build a TakeDamage message
+     (raw wire shape `{id, len, inflictor = DAT_0050b9ac global, zone −1,
+     the Damage verbatim}`) and Dispatch it AT THE OTHER — the mech-vs-mech
+     ram. Use our `Entity::TakeDamageMessage` ctor as the weapon path does.
+
+3. **The CulturalIcon branch** (GUID @0x4e7038):
+   * `stopping = other->instanceFlags & 0x8000` (the StoppingCollisionVolume
+     flag, entity+0x28).
+   * Same separating gate against a ZERO other-velocity (icons are static).
+   * Dispatch the crunch TakeDamage at the icon (crushable props have no
+     zones → their base handler no-ops it, but the dispatch is real).
+   * If NOT stopping: `damage->damageAmount = 0.00123f` (bits 0x3aa137f4) —
+     the WALK-THROUGH SENTINEL. The caller (the response block in Simulate)
+     must treat exactly 0.00123f as "the move stands": restore the post-snap
+     saves, zero the amount, feed the gyro crunch (torque 0.4 along the
+     normalized force, upward impulse 0.2 — binary @4aa81e/@4aa86c).
+
+4. **The material tail**: the elasticity/friction OUT values from
+   StaticBounce are compared against `_DAT_004ac048`; sets
+   `collisionTemporaryState` (+0x44c) to 1, or 2 when both match and the
+   state was 0. The state is pushed into the `collisionState` StateIndicator
+   once per frame by the response block (0→1 fires the impact sound). Needs
+   the `_DAT_004ac048` constant identified before writing (likely the
+   default-material coefficient — check the literal pool at 0x4ac048).
+
+## Two hazards the donor fought that DO NOT apply here
+
+* **Compounded StaticBounce reflections**: our engine's
+  `Mover::ProcessCollisionList` (MOVER.CPP:1203) restores
+  `worldLinearVelocity = initial_velocity` and position PER CONTACT before
+  each ProcessCollision call, and averages the results. The port's
+  `frameEntryWorldVelocity` economy guard has no counterpart hazard in this
+  engine — do not import it.
+* **The AlarmIndicator ODR split**: a port-only problem; our tree reads
+  `MovementMode()` / the alarms directly.
+
+## Already re-hosted in Mech::Simulate (move here when mech4.cpp exists)
+
+| piece | binary | landed |
+|---|---|---|
+| ground probe + snap | @4aa630–4aa6cc | 5.3.98 |
+| frame rejection + crash self-damage + knockdown | @4aa6cf–4aab0b | 5.3.99 |
+| collision economy divert + distributor | @0x4a0368 → @0049ffcc | 5.3.100 (in the damage hub, where the binary has it) |
+| status→mode limp promotion | master perf (un-decompiled) | 5.3.97, STAGED |
+| forwardCycleRate re-pick (ground/airborne) | IntegrateMotion head | partially — the ctor sets it once; the per-frame re-pick needs +0x3f4 |
+
+## Still to decode
+
+* `FUN_004a9b5c` — the master performance. Ghidra never decompiled it
+  (objdump territory). Owns: the turn-in-place dispatcher (arms state 4 on
+  both channels in lockstep), the status→mode promotion, likely the
+  `runSpeedMax` (+0x7a0) writer, the gyro rumble timer.
+* `@004ab430` ReplicantPerformance (1432B) — the replicant-side interior.
+* `@004ab1c8` IntegrateMotion — decoded (see 5.3.95 notes) but only
+  partially re-hosted: the local-velocity/orientation integration and the
+  dead-reckon latency fold (+0x778/+0x77c) remain. **`+0x598` is a Point3D**
+  (its replicant branch writes it with a vector subtract), so
+  `motionEventName` in MECH.HPP is mistyped as CString — fix when this
+  lands.