Persist project-cache snapshot after CLI indexing

[justrach]

Summary

• persist a central ~/.codedb/projects/<hash>/codedb.snapshot after normal CLI scans
• keep repo roots clean: codedb /path tree does not create /path/codedb.snapshot
• reuse the central snapshot writer from snapshot mode and truncate project.txt when refreshing
• add regression coverage for ProjectCache loading a project from the central snapshot cache only

Why

codedb /path tree printed indexed, but a later MCP call with project=/path failed with SnapshotLoadFailed because only side indexes were persisted. After this change, CLI indexing and MCP project loading agree on the cache contract.

Validation

• zig build test
• zig build -Doptimize=ReleaseFast
• smoke: fresh throwaway project, run codedb /path tree, verify no root snapshot, verify central snapshot exists, verify MCP codedb_tree, codedb_symbol, and codedb_status load project=/path

[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	493620	494893	+0.26%	+1273	OK
codedb_changes	54438	52571	-3.43%	-1867	OK
codedb_deps	8582	8602	+0.23%	+20	OK
codedb_edit	5432	6379	+17.43%	+947	NOISE
codedb_find	61958	60986	-1.57%	-972	OK
codedb_hot	97144	98321	+1.21%	+1177	OK
codedb_outline	262401	264858	+0.94%	+2457	OK
codedb_read	87879	87649	-0.26%	-230	OK
codedb_search	175079	176923	+1.05%	+1844	OK
codedb_snapshot	245951	259044	+5.32%	+13093	OK
codedb_status	214900	214953	+0.02%	+53	OK
codedb_symbol	59804	59691	-0.19%	-113	OK
codedb_tree	43825	42190	-3.73%	-1635	OK
codedb_word	70207	67968	-3.19%	-2239	OK

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid blocking CLI scans on full project-cache snapshot write

This new unconditional call runs after every non-snapshot cold scan, so commands like tree and search now synchronously generate a full snapshot before returning. On larger repos (especially once in-memory content caching is exceeded), writeSnapshot re-reads uncached files from disk while writing the CONTENT section, effectively adding a second filesystem pass and causing a significant regression on the benchmark-critical indexing path (likely above the 10% threshold). Consider persisting this cache asynchronously or gating it so normal CLI scans do not pay full snapshot-write cost.

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