refactor(natives): split grep and fs_cache into responsibility modules

[flora131]

Splits the grep and fs_cache native modules into responsibility-named submodules, matching the
subagent_control convention already used in crates/atomic-natives.

The mechanical part_NN.rs line-count splits and their include!(...) splicing are replaced with real
mod submodules plus explicit use/pub use wiring.

Pure reorganization: the generated N-API surface in packages/natives/native/ is unchanged, verified with
git diff --exit-code after a native rebuild.

Commits:
f899beb refactor(natives): re-export grep submodule surface from grep.rs
5c1700d refactor(natives): split grep and fs_cache into responsibility-named submodules

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Greptile Summary

This change replaces generated source-fragment inclusion with named Rust submodules for native grep and filesystem-cache behavior while retaining the existing crate and N-API interfaces.

The compiled native addon was built from both the pre-change and updated revisions. The same runtime checks exercised search, hasMatch, filesystem grep, and invalidateFsScanCache; both revisions returned identical normalized results. The observed filesystem search totals changed from 1 to 2 only after explicit cache invalidation, as expected.

Confidence Score: 5/5

T-Rex Logs

What T-Rex did

• T-Rex ran the native export runtime-check script on both the base addon (pre-change) and the HEAD addon (updated) to compare their export surfaces.
• T-Rex observed consistent results across both runs, including search.matchCount of 2, matching positive and negative hasMatch results, and grep totals progressing 1 → 1 → 2 around cache invalidation, with the same first.txt and second.txt paths and identical normalized runtime output.
• T-Rex validated that the exercised native export surface and behavior were preserved between the base and HEAD addons.
• T-Rex captured and documented the exact commands, working directories, and exit codes used for the two runs.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "refactor(natives): re-export grep submod..." | Re-trigger Greptile}