feat(worker-build): support split WebAssembly debug info

[logan-gatlin]

Summary

• Add split-debug-info profile support to worker-build and enable it in workers-rs templates.
• Preserve DWARF through compilation and wasm-opt, then emit standards-compliant *.debug.wasm external DWARF sidecars.
• Strip runtime .debug_* sections and add one relative external_debug_info reference encoded as a WebAssembly name.
• Keep sidecars through staged and legacy output moves while removing stale sidecars when splitting is disabled.
• Update vendored wasm-bindgen to Walrus 0.27.2, fixing malformed DWARF that caused Binaryen 130 to abort.
• Document the filesystem contract and remaining Wrangler, upload API, workerd, and observability integration work.

Testing

• cargo test -p worker-build (21 tests)
• cargo clippy -p worker-build --all-targets -- -D warnings
• cargo fmt -p worker-build -- --check
• Rust 1.93 Axum template build through Binaryen 130, run twice with identical runtime and sidecar SHA-256 values
• wasm-tools validate on both the runtime and external DWARF sidecar
• Section inspection confirms one runtime external_debug_info, no runtime .debug_*, and DWARF sections in the sidecar

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 2 untouched benchmarks

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_{Comparing t3code/debug-wasm-source-maps (5b37e5f) with main (78dc0cd)¹}

Footnotes

1. No successful run was found on flagship-per-mono-generics (8be4ab2) during the generation of this report, so main (78dc0cd) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[logan-gatlin]

Currently this will do nothing. Needs a workerd follow up to accept wasm debug sidecar files in addition to JS sourcemaps

[guybedford]

What is the benefit of splitting the debug information for Workers, over publishing a binary with the debug section included?

[logan-gatlin]

What is the benefit of splitting the debug information for Workers, over publishing a binary with the debug section included?

Javascript source maps do not count towards the bundle size limit, and do not impact cold start time. The idea is to put wasm debug info in this separate bucket so it gets the same benefits as js