feat(fff-c): add stable C accessor functions for external FFI consumers (#395)

* feat(fff-c): add stable C accessor functions for external FFI consumers

* fff-c: add accessor unit tests (8 tests, 0 warnings)

Tests cover:
- Null-pointer guards: every getter returns its zero-value (0 / false /
  null) when passed a null pointer — no UB, no crash
- Data correctness: FffFileItem, FffGrepMatch, FffSearchResult and
  FffGrepResult getters return exactly the values stored in the struct

All 8 tests pass clean under Rust 2024 edition (no unsafe-op warnings).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* style: apply cargo fmt formatting

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Ionut Adrian Ciolan <iadrian@ciolan.net>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people