[M1-S10] Convert Tiled collision layers into collision.json #92

Problem

Lex draws collision by hand. Authoring polylines in a text file is slow and error-prone; Tiled (free) draws polylines, boxes and points on top of the play layer and exports JSON.

Acceptance criteria

• python scripts/collision_from_tiled.py <export.tmj> writes art/stages/<region>/<chunk>.<state>.collision.json in the schema in docs/13
• Object layers are named after the legend: walk, slide, one-way, ledge, climb, solid, no-go, hazard, interact, spawn, glide-lift
• Fails loudly on an unknown layer, an open box, or a point outside the 2160×720 chunk
• Unit tests with a small hand-made Tiled export

Notes and dependencies

Contract: 13 – Production pipeline. Unblocks Lex's annotation (M1-S7) and the loader (M1-S6). Suggested owner: Vatsal (player and combat code).

Verification

Round-trip a Tiled file of one Rusty Creek chunk; the debug overlay matches the drawing