fix(build): isolate React into stable vendor chunk to prevent HMR crashes

[austin-liminal]

Summary

• Adds manual_code_splitting config to the client bundler to force react and react-dom into a dedicated chunk-vendor-[hash].js chunk, preventing React from being co-located in shared app chunks
• Switches shared chunk filenames from build-ID hashing (chunk-[name]-{buildId}.js) to rolldown's content-based [hash], so the vendor chunk keeps a stable filename across HMR rebuilds and the browser reuses the cached ESM module

Without this fix, HMR would re-import a shared chunk containing React, creating a fresh React instance while the mounted component tree still referenced the old one — causing TypeError: Cannot read properties of null (reading 'useState').

Test plan

• All 211+ unit/integration tests pass
• All 29 E2E tests pass (including HMR rebuild test)
• Coverage at 66% (meets threshold)
• Clippy clean, zero warnings
• Manual verification: run cargo run -- dev --root fixtures/basic, edit a page file, confirm no React dispatcher crash on HMR

🤖 Generated with Claude Code

Note

Isolate React into a stable vendor chunk to prevent HMR crashes in client bundle builds

• Adds manual code splitting in client_bundle.rs to place react and react-dom into a dedicated chunk-vendor-[hash].js chunk, keeping React stable across rebuilds.
• Fixes the chunk filename template from {hash} to [hash] to align with rolldown's content-hash interpolation syntax.
• Adds a test in vendor_chunk_tests.rs asserting that the vendor chunk is emitted, contains React symbols, and is referenced by page entries rather than inlined.
• Behavioral Change: client bundles now emit an additional shared vendor chunk; any downstream asset manifest consumers must handle the new shared_chunks entry.

^{Macroscope summarized 3faa195.}

[macroscopeapp]

Approvability

Verdict: Needs human review

Unable to check for correctness in e5456b6.

No code changes detected at 3faa195. Prior analysis still applies.

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