feat: implement safe leave, profile reset, and full game reset semantics

PLANS.md

@@ -1557,7 +1557,7 @@ At completion, provide the reference sheet, screenshots of the mobile and tablet
 
 ## Milestone 021 — Define and implement safe leave, profile reset, and full game reset semantics
 
-**Status:** `READY`
+**Status:** `DONE`
 **Depends on:** Milestone 020
 
 ### Objective
@@ -1646,11 +1646,22 @@ Add host tests covering at least:
 
 When all criteria pass, set Milestone 021 to `DONE`, append its execution record, and change Milestone 022 from `BLOCKED` to `READY`. Do not start Milestone 022 in the same task unless explicitly requested.
 
+### Execution record
+
+- Date: 2026-08-30
+- Board model and revision: ESP32-C6FH4 QFN32, revision v0.2.
+- Toolchain and library versions: PlatformIO Core 6.1.19; `espressif32` 7.0.1; ESP-IDF 6.0.1; `esp_littlefs` 1.20.4.
+- Result: PASS.
+- Evidence: Added bounded `POST /api/session/leave`, `POST /api/session/profile-reset`, and player-authorized `POST /api/game/reset` commands. Leave/profile reset release only the authenticated session; an active player departure safely creates a new lobby game identity while preserving other valid sessions. Full reset cancels bot work, clears all sessions, boards, mode, bot state, rematch approvals, match and cumulative statistics, and command queue, then creates a fresh game identity and recovery generation. Invalidated HTTP state polling receives stable `SESSION_INVALIDATED` recovery metadata. Authenticated WebSocket owners receive a minimal `{type:"reset"}` notification without board or token data and are invalidated before any role-safe state can be serialized.
+- Measurements: Added lifecycle coverage for spectator/player leave and full reset, HTTP coverage for idempotent leave/reset and invalidated polling, and WebSocket coverage for minimal reset notification/invalidation. `make -C test/host run` passed all ten suites. `node --check data/app.js` and the 12 browser/unit tests passed. `pio run -e esp32-c6-devkitm-1 -t buildfs`, `pio run -e esp32-c6-devkitm-1`, and `git diff --check` passed. Firmware uses 39,604 / 327,680 B RAM (12.1%) and 1,020,806 / 2,097,152 B flash (48.7%).
+- Issues or deviations: No firmware upload or physical multi-client recovery exercise was performed. Milestone 022 owns the browser controls and confirmations; none were added here.
+- Next action: Milestone 022 is READY. Do not start it unless explicitly requested.
+
 ---
 
 ## Milestone 022 — Add the always-available recovery menu and child-safe confirmations
 
-**Status:** `BLOCKED`
+**Status:** `READY`
 **Depends on:** Milestone 021
 
 ### Objective