docs(user): modifier profiles user guide

docs/adr/modifier-profiles.md

@@ -344,3 +344,32 @@ requiring no changes to the registry contract itself.
   and incompatible with deterministic module load ordering.
 
 ---
+
+## Implementation Retrospective
+
+### Deviations from ADR
+
+- **ADR-3 hot-swap timing**: Applied immediately on receipt rather than at
+  an explicit turn-boundary gate. WS message serialization per-socket
+  ensures a client's own move and profile swap cannot interleave. True
+  turn-boundary queue deferred to T2.
+- **ADR-6 check 4 (DEADLOCK)**: Deferred to T2 — requires session
+  simulation which introduces a circular dependency with the engine.
+- **ADR-3 host authority**: Profile swap allowed from host (white player)
+  only in T1. Two-player consent model deferred to T2.
+- **Zod v3/v4 mismatch**: Server pins Zod v3; chess package moved to v4.
+  Schemas mirrored locally in the server package with a compile-time
+  key-parity guard.
+
+### Discovered patterns
+
+- `MODIFIER_REGISTRY` mirrors `PRESET_REGISTRY`/`LAYOUT_REGISTRY` exactly —
+  confirmed the side-effect import pattern scales cleanly.
+- `__modifier-profile-integration__` pseudo-preset registered by
+  `applyProfileToSession` integrates `CaptureFlags` and
+  `DirectionAdditions` into the existing hook pipeline without modifying
+  `engine.ts`.
+- `reconcileProfileSwap` uses retract-then-reapply semantics — idempotent
+  and straightforward to reason about.
+
+---