feat(compositor): the Linux pass of shader effects

[EtienneLescot]

Summary

The Linux pass of agent-written shader effects: the wgpu compositor now draws them, as the Windows one does since #1199. This is section L.4 of docs/shader-effects.md. Stacked on #1199: it only needs that PR's Rust, not the document of #1202.

• Same behaviour as Windows. Three targets (image, text on a transparent text layer, both), lanes, a 64-entry cache. The export waits for a pending compile; the live view skips the effect until it lands, then recomposes (live.rs now turns that on for Linux too).
• Compilation off the render thread. The WGSL goes to wgpu as it is. A background thread builds the shader module and the pipeline against an explicit layout of the prelude's four bindings.
• One new rule in the contract, on every OS. wgpu panics on a validation error, since no on_uncaptured_error is set. analyze now refuses any @binding the prelude does not declare, with its line, so nothing invalid reaches wgpu. A panic would only kill the compile thread anyway, leaving the effect undrawn.
• check_for_agent translates the effect to SPIR-V on Linux, as wgpu will.
• Docs. The known gap in native-compositor.md now says macOS only; the spec (D.5, C.7, H.2) and rendering-performance.md get the Linux details and the lavapipe calibration.

Without a GPU, effects are expensive. On lavapipe a frame() read costs about 1.4 ms per 1080p frame, 350 times an RTX 4070 Ti: a light effect already doubles the frame time. The tiers keep their order.

Related issue

None.

Type of change

• Feature
• Documentation

Release impact

• No release note needed: nothing is reachable from the app until the document model and the tools land (feat(effects): shader effects in the document, and the agent tools #1202)

Desktop impact

• Linux
• Windows: the new binding rule only
• macOS: the new binding rule only, no pass yet

Testing

On lavapipe under WSL (nix compositor-view shell, Mesa's lvp ICD):

• The six GPU tests of the Windows pass, ported to compositor_linux.rs: region repaint, preview non-blocking then shader_fx_landed, broken / extra-binding / unknown-contract effects leave the frame untouched, colour parameter, lane order, the three targets. Run with --nocapture: none skipped.
• cargo test -p openscreen-compositor --lib --tests: all green (479 unit tests, plus the integration tests).
• shader_fx_tier_calibration in release (OPENSCREEN_FX_BENCH=1) for the numbers above.

On Windows: cargo test -p openscreen-compositor --lib shader, 28 tests green.

Not run: the app end to end on a Linux desktop, and an integrated GPU.

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[EtienneLescot]

Integrated into integration/shader-effects as a607e20: the same change, replayed on current main. GitHub closes, rather than marks merged, a PR whose head is already in its base. It reaches main through #1210.