[M0-S14] Export stages for the game: stage.json manifest #109

Problem

The stage loader (#24) reads the docs/13 stage manifest game/public/assets/stages/<region>/stage.json, but nothing writes it yet. The layer plates and the collision files live under art/stages/, which the game can't fetch at runtime.

Acceptance criteria

• python scripts/export_stage.py [regions] writes game/public/assets/stages/<region>/stage.json: chunk ids with grid x/y, and per chunk and state the layer files in draw order with their z and scroll factor, plus the collision file per state
• It copies the layer plates and collision files next to the manifest. Shared plates (Rusty Creek's right chunk reuses the left one's) are copied once.
• The output is generated, not committed (no 18 MB of byte-identical copies). The standard library is enough, so the game's dev and build scripts can run it.
• --verify rebuilds every state from the exported files, in manifest order, and matches the region's composed art pixel for pixel
• Rusty Creek (3 chunks × clean/anomaly) and the Hidden Garden (1 chunk) export

Notes and dependencies

Contract: 13 – hand-over contracts. Unblocks #24 and #34. Owner: Alexander (lane A).

Verification

--verify passes for both regions; unit tests in scripts/test_export_stage.py.