Port Bun test reporter matrix

Tracks a focused tranche of #66.

• Run the byte-identical upstream js/bun/test/bun_test.test.ts instead of rewriting it to a native TODO.
• Model the nested reporter snapshot, including failures, skips, TODOs, assertion counts, hooks, timeouts, and done/promise diagnostics.
• Preserve cross-file snapshot safety failures.
• Cover multifile preload scheduling and --only behavior.
• Add a permanent exact-count regression.

Validation before publication:

• bun_test.test.ts: 5/5 passed, 0 failed/unsupported/TODO
• expect-assertions.test.ts: outer integration passes while recognizing all 5 intended fixture failures
• full home_test: 1,083 tests, 1 expected skip, 0 failures

Completed in 4d8c9cf (feat(test): port bun reporter matrix) and pushed directly to main.

Final validation:

• byte-identical bun_test.test.ts: 5/5 passed, 0 failed/unsupported/TODO
• assertion-failure subprocess companion: outer integration passed and recognized all 5 intended fixture failures
• permanent exact-count regression: passed
• full home_test: 1,083 tests, 1 expected skip, 0 failures