fix: preserve trailing semicolon on nested at-rule statements

[kernoeb]

Summary

Fixes #322.

serializeStylesheet's compress path stripped the trailing ; of any container whose raws.semicolon was set, on the assumption it terminated a redundant declaration body. For a nested at-rule statement (e.g. @layer parent { @layer a, b; }) the ; is the statement terminator — stripping it yields invalid CSS, and browsers then bail on subsequent rules in the parent at-rule.

The fix narrows the strip to cases where the parent's last child is a decl, leaving nested at-rule statements untouched.

Before

@layer parent {@layer a, b}@layer parent.a {.foo{color:red}}

After

@layer parent {@layer a, b;}@layer parent.a {.foo{color:red}}

Verification

• New serialize.test.ts cases:
  • Preserves ; on nested at-rule sub-layer declaration (the bug pattern)
  • Still strips the redundant ; inside an at-rule whose last child is a declaration (e.g. @font-face) — locks in that the new guard doesn't over-restrict
• Ran the serializer end-to-end on real-world bundles before/after the patch:
  • Bootstrap 5.3 minified (232 KB): output byte-for-byte identical
  • Vuetify 4.0 unminified (652 KB): only 3 character differences in the whole bundle, all of them being the ; previously stripped from @layer vuetify-core { @layer reset, base; }, @layer vuetify-utilities { …; @layer theme-foreground; }, and @layer vuetify-final { @layer transitions, trumps; }. Both outputs round-trip through postcss.parse without error.

Test plan

• pnpm --filter beasties exec vitest run — 56/56 pass (2 new)
• pnpm lint
• pnpm test:types

[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

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_{Comparing kernoeb:fix/nested-at-rule-semicolon (896f320) with main (3e5e694)}