qa: define WebGL runtime lifecycle evidence contract

docs/verification/WEBGL_LIFECYCLE_CONTRACT.md

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+# WebGL Runtime Lifecycle Contract
+
+Status: approved implementation contract
+Approved: 2026-07-30
+
+## Required behavior
+
+- A failed or unavailable WebGL context exposes a visible fallback and an equivalent screen-reader message.
+- `webglcontextlost` prevents default browser disposal behavior, pauses rendering, and records a bounded diagnostic event.
+- `webglcontextrestored` rebuilds renderer-owned resources and resumes only after successful reconstruction.
+- In-flight shader or asset requests are cancellable on replacement and unmount.
+- Reduced-motion mode avoids continuous decorative motion while preserving a meaningful rendered state.
+- Unmount cancels animation frames, listeners, requests, observers, and GPU resources.
+- Healthy-path shader appearance remains unchanged.
+
+## Browser evidence matrix
+
+| Scenario | Expected evidence |
+|---|---|
+| Null context | Fallback visible; accessible status present; no uncaught error |
+| Context loss | Frame scheduling stops; diagnostic event emitted |
+| Context restoration | Resources reconstructed; canvas becomes nonblank |
+| Unmount | No later frames; no listeners; no unresolved request |
+| Reduced motion | Stable meaningful frame; no continuous decorative loop |
+| Healthy context | Existing visual snapshot remains within approved tolerance |
+
+## Test boundary
+
+Allowed paths:
+
+- `src/components/ShaderCanvas.tsx`
+- `tests/webgl/**`
+- `docs/verification/WEBGL_LIFECYCLE_CONTRACT.md`
+- `.github/workflows/webgl-browser-evidence.yml`
+
+Forbidden scope includes shader redesign, token changes, pricing, entitlement, dependency migration, and unrelated visual work.
+
+## Approval gate
+
+The PR may move from draft only when browser evidence covers every scenario above and the healthy visual snapshot has no unexplained delta.