[xus1novv]

Problem

During a parallel cold scan, all worker arenas remain allocated until every word-index shard has been merged. At the end of the merge, the completed destination index and all source shards coexist in memory.

Release each worker arena immediately after its shard has been copied into the destination index. The existing cleanup path still handles the current and remaining workers when a merge fails. This reduces the merge-phase peak; it does not impose an overall indexing memory limit or establish that it resolves every large-repository OOM.

Evidence

• In a real incident on 2026-10-28 at 19:40:56 (UTC+05:00), Linux terminated a codedb process after reporting 29,776,092 KiB of anonymous resident memory (approximately 28.4 GiB / 30.5 GB). This is an observed OOM, not evidence that this change alone prevents that incident.
• Pinned Zig 0.17.0-dev.813+2153f8143, Linux x86_64, ReleaseFast, eight scan workers.
• A synthetic cold-index fixture with 128 text files of 512 KiB each used 450,564 KiB peak RSS on the base commit and 388,116 KiB with this change (one measured run of each, using /usr/bin/time; processes capped at 1 GiB address space).
• The new memory-lifetime regression failed on the original merge order (expected 0, found 1586 bytes retained) and passes after the change. A second regression covers merge failure and caller cleanup.
• zig build test-index -Dtest-filter='watcher: parallel word-index shards match sequential' passes.
• zig fmt --check src/watcher.zig and git diff --check pass.

Verification limitations

• zig build test -j1 fails in this environment on two watcher notification tests, repeated across test binaries. The same two tests fail in an unmodified checkout of the base commit.
• The MCP end-to-end suite passes 75/76 scenarios; the semantic index scenario fails with AnnDirectorySyncFailed on both the base and modified binaries in this environment.

[github-actions]

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Please retarget this PR via Edit → base branch to the active release/* branch shown in the repository branch list.

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	⚠️ Failed 2026-10-29T07:07:21.189471Z We couldn’t complete this review. Comment @codex review to try again.	88494ba	Draft marked ready

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