perf: shrink classic production browser output

[ScriptedAlchemy]

Summary

Revives the classic-mode half of #85, which was closed pending the React Router 8 work. Classic production builds now set:

• optimization.mangleExports: 'size'
• optimization.usedExports: 'global'
• output.chunkFilename: 'static/js/async/[id]-[contenthash:16].js'

Classic mode resolves route modules through the browser manifest by chunk, so export names are not part of its runtime contract. RSC mode is untouched and keeps mangleExports: false / usedExports: false, because Flight resolves client references by export name. Development builds are unchanged.

Users who want readable export names in a classic production build can set the two optimization flags back in the function form of tools.rspack, which runs after plugin defaults are merged.

Measured

Gzipped client JS, built from this branch versus 0.6.1:

Example	Before	After	Delta
default-template	111.0 kB	110.4 kB	-0.5%
epic-stack (client output)	336.1 kB	332.1 kB	-1.2%

Smaller than #85's synthetic numbers, which is expected: the synthetic fixtures had far more routes and re-exports to mangle. The epic-stack example's node build fails on main for an unrelated reason, so only its emitted client output was measured.

Verification

• pnpm typecheck, pnpm exec rstest run (all files pass)
• New tests/index.test.ts cases: classic build enables the flags and chunk filename, classic dev leaves them undefined, RSC build still disables mangling

🤖 Generated with Claude Code

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-03T23:35:04.933014Z	526634d	PR opened

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[pkg-pr-new]

Open in StackBlitz

pnpm add https://pkg.pr.new/rsbuild-plugin-react-router@526634d

commit: 526634d

[github-actions]

Benchmark results

Case	Base	Head	Delta
build-256-ssr	1274.8 ms	1248.9 ms	-2.0%
dev-48-ssr	584.3 ms	583.5 ms	-0.1%

[codspeed-hq]

Merging this PR will not alter performance

✅ 1 untouched benchmark
⏩ 2 skipped benchmarks¹

---

_{Comparing perf/classic-production-output-size (526634d) with main (cabf4b9)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (0963271) during the generation of this report, so cabf4b9 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩