perf(build): reuse prepared dashboard gzip set across build-script reruns

[Copilot]

Summary

• Rust-only edits rerun build.rs even when the dashboard bytes are unchanged. Every rerun re-gzipped, round-trip decoded and rewrote every asset, because the cached frontend path still went through from_sources.
• Compressed payloads are now stored as an immutable set per (bundle digest, compression settings). Warm reruns validate and reuse that set instead of compressing again.

Motivation

The frontend build was already cached, but the dashboard embedding step was not. A "frontend cache hit" still paid for Compression::best, a decode, and unconditional file writes.

Changes

• Prepared set: dashboard-gz/<digest>-gzip-best-v1/ holds the gzip payloads and a manifest.tsv.
  • The manifest has one line per asset: encoding, sha256 (gzip entries only), relative path.
  • The directory name carries the settings version. Changing the gzip level or format means bumping DASHBOARD_GZ_SETTINGS.
• Warm path (read_prepared_set):
  • It checks that the manifest matches the asset list exactly and that each gzip payload's sha256 matches.
  • It does no compression, no source reads and no writes.
  • A Rust-only change reuses the set because the directory name depends only on the digest and settings.
  • A changed dashboard asset changes the digest, so it gets a new set.
• Cold path (publish_prepared_set):
  • It compresses each eligible asset once and round-trip checks it. The round-trip check now runs only here, not on every cache hit.
  • It stages everything under .staging-<digest>-<pid> and publishes with a single directory rename, so no build sees a partial set.
  • If a concurrent build wins the rename, the loser re-validates the existing set and uses it.
• Corrupt, truncated or foreign set: it is removed and rebuilt. A directory existing is never treated as valid.
• Unchanged behaviour:
  • Identity is still used for fonts and images, for empty files, and for any file where gzip is larger.
  • MIME type and encoding metadata are the same as before. The MIME mapping is now dashboard_content_type.
  • The generated glue was already written only when its bytes differ.

Test plan

• The affected Bazel targets pass
• bazel build //... passes when the change crosses package boundaries
• Tested manually (describe below if applicable)

Not yet verified. Bazel was unavailable in the authoring environment, so this change has only been rustfmt-checked and has never been compiled or run.

Still to do:

• Build and tests: build //crates/tracedecay-cli:build_script_build and run the existing bundle digest, cache-tag and packaging tests.
• Measurements: the issue asks for warm-rerun numbers on the shipping path (compression calls, bytes written, CPU/wall). None are included here.
• Regression tests: none added. A direct test would be a warm rerun with unchanged dashboard bytes that performs no gzip and writes nothing, plus one corrupted-payload case.
• Open scope item: the issue's preferred step of moving gzip into the Bazel asset-production action is not done. This change only makes the build-script path reusable.

Checklist

• CHANGELOG.md updated (under [Unreleased] if no version bump)
• No secrets, credentials, or .env files included
• Breaking changes documented (if any)

• Fixes perf(build): stop recompressing unchanged dashboard assets on build-script reruns #4342

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