fix(rendering): keep bloom's glow purely additive over lower layers

[stormmuller]

Summary

The bloom composite set the target's alpha to max(scene.a, bloom.a * u_intensity). That made the halo partly cover whatever was presented beneath the camera's render target, such as a background camera's layer, and replace it with glow color. A yellow glow over a blue background reduced the blue.

Render targets are presented as premultiplied alpha (ONE, ONE_MINUS_SRC_ALPHA), so the glow only needs to be added to the color. It still shows past the sprite's silhouette, over fully transparent pixels.

• bloom-composite.frag.glsl: now outputs vec4(scene.rgb + bloom.rgb * u_intensity, scene.a). The comment that described the old alpha blending is rewritten.
• bloom-threshold.frag.glsl: the threshold pass's alpha (the averaged contribution) had no use left, so it now writes 0.
• New e2e scene and spec, bloom-over-background: a bloomed foreground layer drawing a yellow sprite is presented over a background layer cleared to Color(0.1, 0.3, 0.8). The spec reads back the row next to the sprite and checks two things: the glow raises red past the sprite's edge, and blue never falls below its value far from the sprite. With the old shaders the spec fails, with blue dropping from 204 to 33 next to the sprite. With the fix it passes.
• Docs (rendering/bloom.md) and AGENTS.md's premultiplied-alpha section updated to match.

One edge case: if a bloomed camera is the only layer, cleared to transparent and presented straight onto the canvas, its glow pixels now have color but alpha 0. Chromium composites these onto the page as added light.

Related issue(s)

Fixes the reported defect "010: Bloom's halo is partly opaque, so it dims the layers behind it instead of only adding light".

Verification checklist

• npm run check-types passes with 0 errors
• npm test passes (1731 tests)
• npm run lint passes with 0 errors (2 existing TODO warnings)
• npm run cspell passes with 0 errors
• npm run check-exports passes (after npm run build)
• Any new/changed public API is exported from the module's index.ts: no public API changed
• Documentation under /documentation-site/docs/docs is updated (rendering/bloom.md)
• Demos that use bloom (space-shooter, text) were checked after npm run build: docs-site typecheck and build pass, and both pages were loaded in headless Chromium. The space shooter's bullets still glow over the background layer, and the pages showed no errors apart from an external resource that the sandbox network blocked.
• The full e2e suite passes 3× with --repeat-each 3. This was run against the locally installed Chromium build, because the container lacks the build pinned for @playwright/test. The existing check-types:e2e errors in text-effects-overlap.ts are also present on dev.

Changelog

• A bullet has been added under ## [Unreleased] in CHANGELOG.md (#### Fixed)

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https://claude.ai/code/session_016Ldg64ToB3cSB6sDGbpLt8

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