Unflake the iOS/JIT/MCP sim tests that flip under VM load

[obj-p]

Context

The Bazel merge-queue bar (chunk D) runs bazel test //... in a macOS VM. A small set of sim/JIT tests flip pass/fail between runs under VM scheduling jitter — different tests fail each run, which is flakiness, not regressions. The bar now masks this with --flaky_test_attempts=3 (commit on mergequeue-vz-kit); this issue tracks fixing the tests so the retries can eventually be removed.

Observed flaky tests (one or two per run, varying)

1. MCPIntegrationTests — daemon-hosted app server / preview_snapshot: timed out waiting for the first streamed frame (30s). Improved by Fix #292: poll preview_elements for render-readiness instead of a fixed sleep #295 (render-readiness polling) and the 12-vCPU bump, but still timing-sensitive.
2. PreviewsJITLinkTests:IOSPreviewE2ETests.literalEditReSeedsSameObjectWithoutRecompile: a literal-edit hot reload where the snapshot still showed the old string — a propagation race between the edit and the capture.
3. PreviewsJITLinkTests:PreviewsJITLinkTests.perEditRecompileIsBelowWholeModuleCompile: a performance assertion (splitMs < wholeMs) that loses under VM load. Comparing two compile timings is inherently noisy in a shared VM.
4. Earlier (pre-display-fix) bridgeRecordsWindowFrameOnMoveAndResize failed due to the VM's retina logical size; fixed by dropping the display to 110ppi. Listed for completeness — likely resolved.

Directions

• Propagation/timing tests (1, 2): replace remaining fixed waits with readiness polling (as Fix #292: poll preview_elements for render-readiness instead of a fixed sleep #295 did for preview_elements); poll the rendered content for the expected value before asserting, with a generous bound.
• Perf assertions (3): these do not belong as hard gates in a shared VM. Either widen the threshold, convert to informational (log the numbers, do not assert ordering), or tag them to skip in the VM gate. Track the jit_latency numbers as a trend instead of a pass/fail.
• Once the tests are deterministic in the VM, drop --flaky_test_attempts from the bar.

Found while landing the Bazel bar (chunk D). Related: #293 (cache), #294 (parallel sandboxes).