refactor(engine): input as data + live-shadow walk events

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The ECS review of the runtime chain (player → trigger → teleport) found the chain largely clean — event-driven decoupling, per-tick state on the session singleton, stateless reactive systems — with two real violations, both fixed here:

1. Transport-ready rule 3 (InputSourceComponent from day one): PlayerSystem.handleInput read the keyboard directly. Now the new InputSystem is the single device-facing system — it polls the local keyboard each tick and publishes a pure-data intent vector (InputSourceComponent — moveX/moveY normalised, actionHeld) on the session singleton; PlayerSystem consumes the component and never sees a device. Remote peers / replays / AI drivers can drive the player by writing the same component — split-screen and network multiplayer stay a plug-in, not a refactor. Registered before PlayerSystem so the intent is same-frame. Outside runtime mode the intent resets to neutral (mode exit stops the player).
2. WalkOnTileSystem raced live edits: it resolved tile properties from mapData.layers[].sprites — the snapshot that only updates on the persistence fold — while collision reads the live tilemap shadow. A tile painted mid-play didn't change walk events until a save. It now resolves through the shadow (findTileMapForLayer + getSpritesAt), falling back to mapData for layers without a live tilemap (headless contexts).

Specs (both registered in test.mts; engine suite 717 → 761 assertions, green on gjs + node): InputSystem keyboard→component contract incl. runtime gating, diagonal normalisation and neutral-reset; WalkOnTileSystem live-shadow-wins-over-stale-mapData, mid-play edit visibility, empty-tile default.

Docs in the same commit: collaboration-and-multiplayer.md § 5 now records the shipped v1 (singleton intent vector) and keeps the per-player source-descriptor design as the split-screen phase; the stale TODO entry ("nothing emits player-tile-changed today" — false since movement landed) is rewritten to the actually-remaining work.