test: stabilize headless browser CI

[ibgreen]

Summary

• Normalize projected GeoJSON polygon coordinates before comparing headless-browser output.
• Launch Chromium with --disable-dev-shm-usage so the full browser suite does not crash when CI shared memory is constrained.
• Preserve the existing assertions and test coverage.

Root cause

The headless workflow exposed two independent stability problems while testing the unrelated Thrift dependency update in #3463:

1. Chromium projection results can differ at insignificant floating-point precision, making the MVT polygon deep-equality assertion brittle.
2. After that assertion was stabilized, Chromium exited partway through the full suite under CI shared-memory pressure, leaving Vitest with only a browser-disconnect error.

Validation

• The two changed files are identical to stabilization commits 9a970cb7b9 and a06bf852d9.
• Those changes previously passed the full Actions workflow on Add lightweight native decompression #3461.
• The first test: stabilize headless browser CI #3484 run confirmed that the MVT assertion now passes; it progressed to 1,502 passing tests before the browser process disconnected.
• CI is rerunning on this focused two-file draft.