docs(m011): record MVP balance and acceptance evidence

docs/BALANCE_REPORT.md

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+# MVP gameplay balance
+
+This document locks the gameplay configuration evaluated for Milestone 011.
+It is not a claim that the required human playtests have completed; their
+results belong in `docs/PLAYTEST_REPORT.md`.
+
+## Locked configuration
+
+| Parameter | Locked value | Purpose |
+| --- | ---: | --- |
+| Maze dimensions | 128 x 64 cells | MVP map size |
+| Initial vision radius | 3 cells | Current-view-only exploration |
+| Cross-half connection pairs | 3 | Multiple routes between conceptual halves |
+| Extra cycle pairs | 64 | Alternative routes without removing dead ends |
+| Minimum start-to-exit separation | 64 cells | Keeps exits distant from starts |
+| Landmark pairs | 8 | Spatial-memory cues in symmetric positions |
+| Vision-artifact pairs | 4 | Eight one-use artifacts; exceeds the MVP minimum of two |
+| Compass pairs | 1 | One compass for each player's own exit |
+| Movement cooldown | 150 ms | Server-enforced minimum accepted-move interval |
+| Enhanced-vision duration | 10 successful moves | MVP artifact duration |
+| Draw window | 200 ms | Authoritative simultaneous-exit boundary |
+| Match timer | None | Matches end only by defined result conditions |
+
+The values come from `maze32_maze_default_config()` and
+`maze32_game_default_config()`. They are server-owned defaults; browser
+clients cannot alter them.
+
+## Deterministic generation evidence
+
+`test/host/test_maze_generation.c` generates and validates seeds 0 through
+1,023 with this configuration. For every generated map it proves reachability,
+central symmetry, positive cycles and dead ends, at least four cross-half
+connections, paired player-relative route distances, and this balance
+invariant:
+
+```
+start_to_key_distance >= key_to_exit_distance
+```
+
+The test measures shortest traversable routes, not straight-line distance. It
+therefore establishes that finding a key is never made easier by a shorter
+required route than the later exit search. It does not substitute for human
+search behaviour in a maze with fog of war, landmarks, compasses, and
+artifacts.
+
+The most recent recorded sweep (2026-09-08) reported:
+
+| Metric | Observed range across 1,024 seeds |
+| --- | ---: |
+| Cycles | 133..133 |
+| Dead ends | 704..854 |
+| Start to key | 416..1,442 cells |
+| Key to exit | 346..1,244 cells |
+| Start to exit | 102..992 cells |
+| Generation attempts | 1 |
+
+At the 150 ms cooldown, an uninterrupted shortest 416-cell key route takes at
+least 62.4 seconds of accepted-move time. Actual match duration also includes
+route discovery, current-view-only navigation, the exit search, detours,
+interactions, and player decisions. The approximately five-minute target must
+be confirmed by the representative human-playtest procedure; no hard timer is
+introduced to force that outcome.
+
+## Tuning decision
+
+No parameter change is justified before representative timing evidence exists.
+Changing maze density or cooldown solely from shortest-path metrics would risk
+making the target duration less representative of real fog-of-war play. If the
+playtest median materially misses the five-minute target, change only the
+smallest relevant locked value, rerun the 1,024-seed suite, record the new
+range here, and repeat the affected playtests.