gpui: Add frame snapshot oracle and render benchmarks

[mikayla-maki]

Summary

• add an exact, test-support-gated FrameSnapshot covering scene primitives, hit testing, dispatch/focus state, cursor and tab requests, deferred draws, and accessibility updates
• add a deterministic dual-window frame oracle with a walk-vs-walk storybook and property-test scaffolding
• add GPUI frame-production Criterion benchmarks for layout, scene assembly, presentation proxies, and a scripted 300-frame workload
• report layout/prepaint and scene-assembly phase histograms through the existing GPUI benchmark report
• document snapshot canonicalization and extending storybook coverage

Canonicalization

Oracle windows share the test context's text system and sprite atlas. Scene order remains semantic, and primitive floating-point fields are compared by their IEEE-754 bit representation.

The oracle work also moved ordinary animation timing onto the GPUI executor clock so deterministic tests do not leak wall-clock time.

Validation

• walk-vs-walk storybook passed across 100 scheduler seeds
• generated walk-vs-walk property test passed with seeds 0, 1, 7, 42, and 99
• GPUI animation tests passed
• cargo check -p gpui --features test-support
• cargo check -p benchmarks --bench gpui_frame_production
• ./script/clippy -p gpui --features test-support
• cargo fmt --all
• git diff --check

Full Criterion sampling and the <5% machine-local variance check remain to be run on the target benchmark machine.

Suggested .rules additions

For GPUI code whose behavior depends on elapsed time, use the context executor's clock (cx.background_executor().now()) instead of scheduler::Instant::now() or Instant::elapsed(). The executor clock follows the deterministic test clock while production executors still use real time.

Release Notes:

• N/A