Use index for filename listing - #127

Routes --files through indexed filename data while preserving filesystem fallbacks.

Changes:

• Adds and maintains the files-extra.bin sidecar.
• Adds server and local-index filename listing.
• Adds watcher synchronization, tests, and documentation.

tgrep-core/src/walker.rs:582

• Metadata failures are treated as “not too large” here, so a binary-extension entry whose metadata cannot be read is added to listed_files without incrementing skipped_error. The server treats skipped_error == 0 as proof that this walk is authoritative, which can publish a stale/uninspectable path in files-extra.bin. Read metadata with the same error handling used for normal files before adding either kind of path.

                let too_large = max_file_size
                    .is_some_and(|limit| entry.metadata().is_ok_and(|meta| meta.len() > limit));
                if !too_large {
                    listed_files.lock().unwrap().push(rel_path);

tgrep-cli/src/serve.rs:2175

• The publication result is discarded after replace_filename_extra_paths has already updated memory and marked the index ready. If the rename fails, an older valid sidecar remains on disk; subsequent refreshes see no in-memory change and never retry, so standalone indexed --files can later trust stale paths. Preserve a pending/dirty state until publication succeeds (and ensure a stale sidecar cannot be treated as current).

    if replace_filename_extra_paths(state, listed_files) {
        persist_filename_extra_paths(state, index_dir);

tgrep-cli/src/search.rs:497

• This checks only the query flags, not the traversal policy used to build the index/server snapshot. For example, an index built with --hidden makes a plain tgrep --files return hidden paths, while one built with a smaller --max-filesize silently omits files the default query admits. Persist the build-time membership settings (hidden/ignore/excludes/size cap) and require compatibility before using either local or server filename data; legacy indexes without that metadata should walk.

fn filename_index_compatible(opts: &SearchOptions) -> bool {