[web] Clear registered typefaces with the Skwasm font collection - #194216

SkwasmFontCollection.clear() recreates the native collection but retains the old SkwasmTypeface wrappers in registeredTypefaces. Clear that map as well, so repeated font load/clear cycles no longer accumulate those references. Add an isolated regression test that registers two fonts under one family and clears/reloads for three cycles.

Fixes #194215. No public API, website documentation, or flutter/tests migration is needed.

Validation on macOS arm64, Chrome 145, Dart 3.15.0-18.0.dev:

• New lifecycle test fails on the base revision and passes with the one-line fix.
• Existing test/ui/font_collection_test.dart: 7 tests pass in chrome-coi-dart2wasm-skwasm-ui.
• Targeted dart analyze, dart format --set-exit-if-changed, and git diff --check pass.
• Tests compile the local Dart sources and use the matching official Skwasm artifacts via felt --gcs-prod. Full engine CI has not run locally.

AI-assisted draft, pending contributor review and CLA confirmation before requesting review. This change removes retained references; no GC timing or performance improvement is claimed.

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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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