Add thirty-second controller replay with scrubbing and free camera

docs/replay.md

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+# Instant replay
+
+The game keeps the last 30 seconds of completed presentation poses in memory.
+Select / View (the small left menu button) opens replay, paused at the newest
+frame. The available duration grows from zero after starting the game.
+
+| Control | Action |
+| --- | --- |
+| Select / View, or B | Exit replay |
+| LT / RT | Scrub backward / forward; trigger pressure changes speed, up to 4× |
+| D-pad left / right | Step one recorded frame |
+| A | Play / pause; playing from the end starts at the beginning |
+| Y | Toggle recorded camera / free camera |
+| Left stick | Move free camera horizontally |
+| Right stick | Look around |
+| LB / RB | Move down / up |
+| Left-stick click | Faster camera movement |
+
+Keyboard: F6 opens/closes replay, arrows scrub, comma/period step frames,
+Space plays/pauses, C toggles camera. Free camera uses WASD, Q/E for height,
+right mouse drag to look, and left Shift for speed.
+
+Replay pauses live gameplay. Exiting returns to that same live state, not the
+scrubbed position. Release the replay controls before resuming gameplay. Escape
+can still open the graphics menu. This is an in-memory viewer, not video export
+or a saved replay format.
+
+## Implementation and verification
+
+`Replay` stores timestamped `presentation::Snapshot` values: root transform,
+complete skin/board pose, camera transform, and FOV. It records completed fixed
+ticks and uses the same bone-local interpolation as live presentation. Camera
+cuts, teleports, and cadence changes are not interpolated across. Scrubbing never
+advances animation graphs, physics, gesture recognition, or stance events.
+
+The replay input system runs after platform polling in PreUpdate, outside the
+gated fixed simulation sets. Replay inputs are discarded from gameplay history;
+the gate remains on input until buttons, sticks, and triggers are released.
+
+Run `cargo test -p skate-game --locked replay::tests` for buffer retention,
+scrubbing, camera mode, discontinuity, input isolation, and simulation gating.
+For a screenshot smoke check, set `SKATE_VERIFY_REPLAY=1` when using the existing
+`--verify <absolute-path.png>` launch option.
+
+The remaining trick hitch and nollie inward-heel/fakie behavior are not changed
+by this feature. Replay allows inspecting their final displayed poses directly.