perf: write output files in parallel and create each directory once - #11680

closes #11670

Summary

bundle_write wrote every output file one at a time and probed (exists) the parent directory before each write. On a large app (~28k output files including maps) that loop is serial syscall latency on the critical path between generateBundle and writeBundle.

This PR:

• validates all filenames for NUL bytes before writing anything (previously an invalid name could leave a partially written outDir)
• creates each distinct parent directory once, instead of probing per file
• writes files in parallel with rayon when all destination paths are distinct
• falls back to sequential writes if two outputs target the same path, preserving last-writer-wins
• reports the first failure in output order, so errors stay deterministic

writeBundle still runs only after all writes complete.

Measurements

Large synthetic Vite app (13k chunks, 16k assets, client build), M4 Pro, profile build, 2 paired runs, native tracing spans. The write loop plus writeBundle hooks (bundle_write minus generate):

* measured together with #11681, #11682, #11683. Those don't touch this phase, apart from the lazy metadata change affecting writeBundle hook object construction. Whole-build wall time is too noisy (±5 s) to isolate this change alone.

Test plan

• cargo test -p rolldown --lib
• cargo test -p rolldown --test integration: no snapshot changes (only env failures: test262 submodule, missing util-deprecate)
• cargo clippy -p rolldown --all-targets

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