test: cover dynamic CSS parity

[ScriptedAlchemy]

Summary

• add e2e coverage for the dynamic CSS ownership scenario from the React Router Vite parity audit
• verify that Rsbuild/Rspack keeps route CSS applied when the same CSS is dynamically imported later
• intentionally adds no plugin runtime hook because the scenario passes with Rsbuild/Rspack CSS handling

Validation

• pnpm build
• pnpm test:core
• cd examples/default-template && pnpm exec playwright test -g "keeps route CSS applied when the same CSS is dynamically imported"
• git diff --check

Notes

Stacked on codex/route-dev-reload-parity. I did not add unstable_setDevServerHooks for critical CSS because Rsbuild extracts CSS files by default and the dynamic CSS e2e passes without it. I also did not add request-error stack trace hook code because no failing repro was established.

[github-actions]

Benchmark Results

Compared PR head 4e6d0f8 against base 6ab91d0.

Total median wall time: 12.35s -> 12.13s (-1.8%, 1.02x speedup)
Compiler ready median: 10.17s -> 9.92s (-2.4%)
Route load median: 2.10s -> 2.14s (+1.7%)

Benchmark	Base total	Head total	Delta	Head ready	Head routes	Speedup	Head RSS p95
large-355-ssr-esm	12.35s	12.13s	-1.8%	9.92s	2.14s	1.02x	-

Profile: large; mode: dev; iterations: 3; warmup: 0.
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