Implement the --json output sink for pdftract grep with JSON-Lines
format (one match per line). Includes MatchEvent, FileOnlyEvent,
CountEvent structs and JsonSink line-buffered writer.
Key features:
- MatchEvent with all fields (path, page_index, bbox, match_text,
span_text, span_confidence, pdf_fingerprint, crosses_spans)
- crosses_spans omitted when false via skip_serializing_if
- NaN/Infinity in span_confidence replaced with null
- page_index is 0-based (machine convention)
- FileOnlyEvent for -l mode, CountEvent for -c mode
- Line-buffered writes with immediate flush
- JSON schema at docs/schema/v1.0/grep-jsonl.schema.json
Closes: pdftract-5ls35
Implement bead 7.8.2: Build the per-search matcher from GrepArgs.
Compile PATTERN into either a literal Aho-Corasick automaton (-F mode,
default) or a regex::Regex (-E mode). Apply -i (case-insensitive) and
-w (word-boundary) wrappers. Provide a uniform Matcher::find_iter(text)
-> Iter<MatchRange> API used by the per-span matcher.
Key changes:
- Add aho-corasick dependency for fast literal matching
- Create grep/matcher.rs with MatchRange and Matcher enum
- Reorganize grep.rs -> grep/mod.rs for proper module structure
- Implement literal mode with Aho-Corasick automaton
- Implement regex mode with regex::Regex
- Support case-insensitive matching in both modes
- Support word-boundary matching (\b anchors for regex, post-match check for literal)
- Comprehensive unit tests for all modes and edge cases
Closes: pdftract-ixzbg
