feat(effects): integrate analytic GTAO ambient composition

[ibgreen-openai]

Goals

• Replace the existing horizon-inspired heuristic with analytic, cosine-weighted GTAO visibility.
• Preserve direct and emissive lighting through an additive, backward-compatible ambient-only composition mode.

Changes

• Integrate signed positive/negative horizon angles per view-space slice and rotate/jitter samples with frameIndex.
• Add composition: 'ambient-only' and export gtaoAmbientComposite, while retaining the existing whole-color composite by default.
• Add reusable deferredAmbientLighting extraction and update the deferred-rendering example to bind isolated ambient radiance.
• Extend WGSL compilation, compositing behavior, pipeline-shape, deferred integration, and documentation coverage.

Verification

• env -u YARN_NO_PROXY corepack yarn lint fix
• env -u YARN_NO_PROXY corepack yarn lint
• env -u YARN_NO_PROXY corepack yarn test-node — 178 passed, 2 skipped.
• env -u YARN_NO_PROXY corepack yarn build — all packages compile.
• env -u YARN_NO_PROXY corepack yarn examples:typecheck — all 31 example workspaces pass.
• env -u YARN_NO_PROXY corepack yarn website:build — production client, server, and search index build.
• env -u YARN_NO_PROXY corepack yarn test-headless modules/effects/test/passes/advanced-effects.spec.ts modules/effects/test/passes/postprocessing-wgsl.spec.ts modules/experimental/test/rendering/deferred-lighting.spec.ts — 8 Chromium/WebGPU tests pass.
• env -u YARN_NO_PROXY corepack yarn test runs 178 node tests and 1,303 headless cases; one unchanged, reproducible existing failure remains in modules/arrow-layers/test/layers/arrow-layers.spec.ts (polygonViewportUniforms missing / polygon storage instances not drawable). The affected Arrow files have no diff from master.