fix(grep): report truthful insufficient-result stats

[AlexMikhalev]

Summary

Fix terraphim_grep structured output on the RlmInsufficient path:

• preserve retrieved chunks and KG concepts;
• derive stats.chunks_returned from chunks.len();
• derive stats.kg_hits from concepts.len();
• add library and real CLI regressions.

This does not change the sufficiency threshold. A one-result search may remain RlmInsufficient; its statistics are now truthful.

Evidence

Strict RED→GREEN:

• RED library: chunks_returned was 0, actual chunks 1.
• RED CLI: stats.chunks_returned must equal chunks.len() (got 0, chunks = 1).
• GREEN focused library: 1 passed, 0 failed.
• GREEN CLI: 1 passed, 0 failed.

Final gates:

• cargo test -p terraphim_grep: 61 tests passed across targets, 0 failed; one doctest ignored.
• cargo fmt -p terraphim_grep -- --check: pass.
• cargo clippy -p terraphim_grep --all-targets -- -D warnings: pass.
• Branch binary smoke: chunks=1 chunks_returned=1 kg_hits=0 sufficiency=RlmInsufficient.
• git diff --check: clean.

Tracking

• Root cause: terraphim/terraphim-ai#3169
• Release wrapper: terraphim/terraphim-ai#3208
• Supersedes the implementation intent of stale-base PR fix(grep): return chunks for release searches terraphim-ai#953 with a clean current terraphim-clients base.

Scope

No version bump, publish, threshold change, dependency update, or unrelated crate change.