Summary

Adds explicit Skwasm artifact selection and browser-test coverage for Safari and Firefox.

The new tests prove which Skwasm artifact was loaded and verify that the run did not silently fall back to CanvasKit.

Why now

#188797 fixed the stock RenderCanvas CSS-size drift observed on Safari, and the corresponding issue #188723 is now closed.

With that specific rendering blocker addressed, the next gap is artifact-level confidence. Safari and Firefox automatically select skwasm_heavy, but existing tests do not prove which runtime files were loaded. This makes it possible to accidentally measure another Skwasm variant, Wimp, or a CanvasKit fallback.

This PR makes that selection explicit and testable.

Changes

• Adds skwasmVariant with auto, normal, and heavy options.
• Adds Felt support for selecting a Skwasm variant.
• Adds non-CI Safari and Firefox Skwasm suites.
• Verifies that the expected Skwasm JavaScript and Wasm files were loaded.
• Verifies that other Skwasm variants and CanvasKit were not loaded.
• Tracks Wimp artifacts separately so Skwasm-only suites do not copy or depend on them.

The default remains auto; existing production selection behavior is unchanged.

Local validation

A three-pass local macrobenchmark comparison used the same SDK and browser harness for CanvasKit and Skwasm. The metric is drawFrameDuration.average; lower is better.

The selected artifact was recorded on every run, and the direction did not change across the three passes.

A controlled variant check also showed why this proof matters: forcing normal skwasm on Safari or Firefox fails during text layout because Intl.v8BreakIterator is unavailable. skwasm_heavy includes the required text-segmentation data and runs successfully.

These are local results supporting the test infrastructure in this PR. They are not a default-enablement or CI-readiness claim.

Scope

This PR does not:

• enable Skwasm by default on Safari or Firefox;
• change the rendering or presentation path;
• add Safari or Firefox Skwasm suites to CI.

Related: #178893, #188797.

Tests

• dart analyze test/skwasm/artifact_loading_test.dart dev/felt_config.dart dev/felt_config_test.dart dev/test_platform.dart dev/test_runner.dart dev/steps/copy_artifacts_step.dart
• dart test dev/felt_config_test.dart
• ./dev/felt test --suite=firefox-dart2wasm-skwasm-skwasm --suite=safari-dart2wasm-skwasm-skwasm test/skwasm/artifact_loading_test.dart