Avoid partial word index rebuilds

[justrach]

What changed

• Prevent fast snapshot restores with capped content caches from rebuilding a silently partial word index.
• Rebuild the word index by streaming files from the project root when the cache is incomplete and a root is available.
• Fail closed with WordIndexIncomplete when only a partial snapshot cache exists and no source root is available.
• Extend the large snapshot regression test to cover both paths.

Validation

• zig build test
• zig build

Fixes the review finding on #329 where a fresh machine with only codedb.snapshot and no readable word.index could miss codedb_word hits after the 1000-file content cache cap.

[github-actions]

Benchmark Regression Report

Thresholds: 10.00% and 50,000 ns absolute delta

NOISE means the percentage threshold was exceeded, but the absolute delta was too small to fail CI.

Tool	Base (ns)	Head (ns)	Delta	Abs Delta (ns)	Status
codedb_bundle	542476	536460	-1.11%	-6016	OK
codedb_changes	60594	57090	-5.78%	-3504	OK
codedb_deps	9910	10030	+1.21%	+120	OK
codedb_edit	6371	6428	+0.89%	+57	OK
codedb_find	67520	68634	+1.65%	+1114	OK
codedb_hot	106009	105689	-0.30%	-320	OK
codedb_outline	286486	293146	+2.32%	+6660	OK
codedb_read	93323	100728	+7.93%	+7405	OK
codedb_search	215383	178500	-17.12%	-36883	OK
codedb_snapshot	264775	284967	+7.63%	+20192	OK
codedb_status	108569	113733	+4.76%	+5164	OK
codedb_symbol	65125	63467	-2.55%	-1658	OK
codedb_tree	46190	50676	+9.71%	+4486	OK
codedb_word	112711	76628	-32.01%	-36083	OK

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Rebuild word index under writer lock to avoid lost updates

rebuildWordIndex now does most of its work without holding the write lock, then swaps self.word_index at the end. In MCP/serve mode, queries can run concurrently with watcher.incrementalLoop updates (commitParsedFileOwnedOutline/removeFile), so updates applied while rebuild is in progress can be overwritten by this final assignment, leaving word_index_complete = true but missing recent file changes until another mutation happens. The previous implementation rebuilt while holding the writer lock, so this is a correctness regression under concurrent query + file-change workloads.

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