feat(game): implement authoritative movement and visibility core

docs/GAME_CORE.md

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+# Authoritative game core
+
+Milestone 006 provides the portable movement engine and view projector. The
+ESP remains the only owner of mutable game state. Object interaction, item
+effects, exits, results, reconnect behavior, and browser controls remain out of
+scope until their later milestones.
+
+## Command processing
+
+Move commands enter a fixed 16-entry FIFO. The server assigns a monotonically
+increasing command sequence when an entry is accepted into the queue, then
+processes entries one at a time. An accepted move crosses exactly one open
+north, east, south, or west passage. Countdown commands, closed-wall moves,
+out-of-order timestamps, and commands inside the configured per-player
+cooldown leave authoritative game state and the state sequence unchanged.
+
+Each player has an independent cooldown clock. Player positions never block
+one another. When an accepted move ends on the other player's cell, that
+command emits exactly one meeting event and advances the meeting sequence.
+Observers cannot enqueue commands.
+
+## Projection boundary
+
+Player projections contain only cells whose Chebyshev distance from the
+player is at most the active radius:
+
+`max(abs(x - player_x), abs(y - player_y)) <= radius`
+
+The projector supports the normal radius three and vision-effect radius six.
+Map boundaries clip the square; walls do not occlude it. Each projection is
+rebuilt from zero, so an opponent or object that leaves the current square is
+omitted instead of surviving as stale state. Observer projections contain all
+maze cells, static objects, and both current player positions.
+
+The serialized view is split into bounded JSON records of at most 512 bytes,
+matching the WebSocket gateway's message bound.
+Every record uses the architecture's versioned envelope and carries the
+authoritative state sequence, role, match phase, meeting sequence, chunk index,
+and chunk count. Cell records contain only the cell index and current passage
+flags. Object records contain kind, owner, variant, and cell index. Player
+records omit the `opponent` member when that player is not currently visible;
+observer records instead carry both positions in `players`.
+
+Static maze objects are projected for visibility testing, but Milestone 006
+does not pick up, consume, transfer, or otherwise mutate them.