fix(templates): stabilize 3D browser shell

[wh000wh000]

结论

Core3d now has a deterministic browser shell: repeated Escape cannot toggle pause twice, completed/failed games cannot be resumed, and favicon loading uses a real relative file instead of inline data.

Product-code evidence

Finding	Generated code	Template gap	Runtime consequence
ESC repeat/end lock	ember-skyway-relics-3d-v1-4/src/main.ts:37	core3d/src/main.ts accepted every Escape	held Escape randomly toggled pause; end overlays could resume simulation
Missing path favicon	v1.4 index.html:3-7 had no icon	core3d had no public/favicon.*	independent browser emitted a 404
Inline workaround	Crystal v1.2 used href="data:,"	no canonical path asset	hid the 404 by embedding data in editable HTML

Change

Path	Change
agent-test/templates/core3d/src/main.ts	Ignore event.repeat; lock Escape after completion/failure
agent-test/templates/core3d/index.html	Reference ./favicon.svg
agent-test/templates/core3d/public/favicon.svg	Small text SVG asset; no Base64/Data URL

The complementary non-Git smoke change adds opt-in --shell-contract: repeated Escape must preserve pause and the icon must be a non-data path returning 200. Default historical 2D/3D smoke behavior is unchanged.

Verification

Gate	Result
Fresh core3d npm ci && npm run build	PASS
smoke --esc-rounds 2 --shell-contract	PASS: WebGL/non-black/errors0, repeat stable, favicon 200
Crystal v1.2 negative	Expected FAIL under opt-in: repeat changed pause and data:, icon rejected
Crystal v1.2 default smoke	PASS
Base64/Data URL scan of new template	0

Risk and rollback

The changes affect only the 3D shell. Phaser templates and default smoke semantics are untouched. Revert 4e4fdf5 for the icon path or ab0c546 for lifecycle behavior independently.

Upstream dependencies: leigest519#47-leigest519#50.