chore(gates): exclude .fixtures.ts modules from changed-line coverage

runtime.fixtures.ts (packages/platform-android/src) sits outside any
__tests__ dir, so vitest's coverage.include picked it up as production
source and the changed-line gate scored its 20 unreachable construction
lines directly, failing at 39.39% against the 70% threshold.

Add '**/*.fixtures.ts' to coverage.exclude (repo-wide convention:
40+ fixture modules, all test support, never production). The file
disappears from lcov and scripts/coverage-changed/model.ts's existing
excluded-path fallback reports it non-gating instead, with no
duplicate classifier needed there.

Planted red (targeted run against the real diff/model, not committed):
BEFORE (file present in lcov, all added lines uncovered): totalLines=110
coveredLines=0 pct=0 passed=false. AFTER (file absent from lcov post-fix):
totalLines=0 excludedTotal=99 excludedReason=excluded-path passed=true.
Confirmed against a real `vitest run --coverage` + `check:coverage-changed`
pass: runtime.fixtures.ts absent from coverage/lcov.info, gate PASS,
99 lines reported under excluded/excluded-path.

Author: thymikee

vitest.config.ts

@@ -242,6 +242,7 @@ export default defineConfig({
       exclude: [
         'src/**/*.test.ts',
         'src/**/__tests__/**',
+        '**/*.fixtures.ts',
         'src/**/*-types.ts',
         'src/**/types.ts',
         'src/sdk/**',