perf(decofile): make in-commit blocks.gen.json regen cache-aware

[guitavano]

Summary

The decofile commit path (commit-coalescer.ts) regenerates a committed .deco/blocks.gen.json inside every save commit. Today it re-fetches every unchanged block via client.getBlobText (bypassing the disk cache) in an unbounded Promise.all. On a large repo (e.g. Granado, ~325 blocks) that's hundreds of uncached GitHub round-trips per save — several seconds of latency before the preview can even load, since the preview reads by commit sha.

This is the general-case (option "a") counterpart to per-repo untracking of the artifact: it helps every repo that keeps blocks.gen.json committed.

Change

Align the in-commit regen with the read path (resolveSnapshot):

1. Read unchanged blocks disk-cache-first + bounded. New shared getBlockText(client, sha) tries the disk cache before GitHub and writes through on a miss; the loop uses mapBounded(..., BLOB_FETCH_CONCURRENCY). The editor's read right before a save already primed those blobs, so a same-replica save is mostly cache hits; a cold replica fetches at most N at a time instead of stampeding GitHub's abuse limiter (the exact reason the read path is already bounded).
2. Prime the merged-doc cache after commit. primeMergedSnapshot(...) stores the just-computed document under the new sha's merged-doc key — byte-identical to what resolveSnapshot produces at that sha (same mergeBlocks, same sort) — so the preview read immediately after a save is a disk hit instead of a fresh tree read + re-merge.

Scope / tradeoff

• Only repos that commit blocks.gen.json enter the regen branch; gitignored repos (the common case) are untouched.
• Known tradeoff (option "a"): the big win is on the warm path (same replica, not yet LRU-evicted). A cold replica still fetches all blocks — but now bounded instead of an unbounded stampede, so it's never worse than before. The deterministic-but-riskier alternative (incremental patch of the prev gen) would break byte-identity with the daemon's canonical merge, so it was not taken here.

Testing

• New e2e (decofile-api.spec.ts): seeds a repo with a stale committed blocks.gen.json (missing a block, pre-edit value), PATCHes a block, and asserts the artifact was fully re-merged (edited + untouched blocks, keys sorted by filename). This branch had zero e2e coverage before. Passing locally against the real server + GitHub stub (1 passed (15.4s)).
• bun run check (apps/api + packages/e2e) and existing decofile unit tests pass; bun run fmt clean.

🤖 Generated with Claude Code

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Summary by cubic

Speeds up saves in repos that commit .deco/blocks.gen.json by making the in-commit regeneration disk-cache-first and bounded, instead of re-fetching every unchanged block from GitHub in an unbounded Promise.all.

• Reads unchanged blocks through the shared getBlockText path with BLOB_FETCH_CONCURRENCY concurrency, mirroring the read path.
• Primes the merged-doc cache for the new commit sha so the preview read right after a save hits disk instead of re-reading the tree and re-merging.
• Only affects repos that commit the artifact; gitignored repos are untouched.
• Adds an e2e test covering the regeneration branch, which previously had no coverage.

^{Written for commit f92ea96. Summary will update on new commits.}