feat(core): expose apply_search_filters as a public API

[githubrobbi]

Summary

Flips `apply_search_filters` from `pub(crate)` to `pub` (plus an explicit
`pub use` re-export, since its containing module is otherwise re-exported at
`pub(crate)`). Visibility change only — no new data or logic. `SearchFilters`
and `DisplayRow` are already fully public types with their whole field list
visible in source today; what's new is the ability to call the existing
~20-axis filter-matching engine from outside `uffs-core` at all.

Motivating use case

A private downstream crate (`uffs-index-private`, planned in the
`uffs-products` repo — see `PRIVATE_RICH_INDEX_DESIGN.md` there) wants to
carry more fields per record than the public `CompactRecord` does
(ChangeTime, USN, security_id, owner_id — all already parsed by `uffs-mft`'s
`StandardInfo` today, just unused downstream), without growing the public
struct's per-record cost for every UFFS user.

It builds its own richer record type, adapts the overlapping fields into a
`DisplayRow` via `DisplayRow::new` (already a public constructor taking only
plain data), and can now run the whole existing filter engine against it
unchanged instead of forking or reimplementing it — then applies its own
small extra-fields filter pass on top for whatever `DisplayRow` doesn't
carry.

Test plan

• `cargo doc` confirms the item is genuinely externally reachable at
  `uffs_core::search::filters::apply_search_filters` (not just visible
  in source)
• `cargo fmt --check` / `just lint-prod` / `just lint-tests` clean
• `cargo test -p uffs-core --lib` — 885 passed, 0 failed