feat: add fts support (#408)
Add BM25-based full-text search with CJK (jieba) tokenization, supporting
query_string and match_string syntax, phrase queries, boolean operators
(AND/OR/NOT/MUST), and hybrid retrieval with existing vector search.
## Core
- BitPacked posting format with block-max WAND pruning
- Tokenizer pipeline: jieba (cut/cut_for_search/hmm/full), whitespace,
lowercase, with extensible pipeline composition
- Query parser: boolean operators, phrase queries, field scoping,
boost, MUST(+) modifier inside OR (ES query_string semantics)
- AST rewriter: dedup repeated terms with linear boost aggregation,
flatten same-type composites, canonicalize OR-with-must_not into AND
wrapper, empty-node propagation, contradiction detection
- FTS reduce/merge integrated into Optimize compaction
- Multi-segment score-descending sort
- Auto-register bundled jieba dict on SDK import
## Performance
- Block-max WAND with cached block_max_info_for (single binary search)
- AVX2/SSE bitpacked encoding with cross-arch scalar fallback
- MultiGet for batch posting retrieval and phrase position verification
- HashSkipList memtable for posting writes
- PinnableSlice zero-copy reads
- Filter pushdown into composite iterators (Disjunction/Conjunction/Phrase)
- Candidate-driven (brute-force) evaluation for selective invert filters
- Precomputed BM25 IDF weights, cached SIMD dispatch pointers
- Shortest-list anchor for phrase position matching
- Single-open per-term posting iterator
## Query
- Tokenize query terms through the same pipeline as indexing
- EmptyNode for zero-token queries (all stop-words / punctuation)
- Backslash unescape after lexing in query parser
- Schema allows collections without vector fields (FTS-only use case)
- Create/Drop Index validates supported index types
- FTS fields disallowed in SQL filter expressions
## Bindings
- C API: fts query params, brute-force ratio config
- Python SDK: FTS search, jieba dict auto-registration
## Internals
- Bypass cppjieba::Jieba to drop KeywordExtractor (~12MB fewer required files)
- Hide tokenizer pipeline from public header (Pimpl-style FtsState)
- ListColumnFamilies to avoid double-open on segment load
- Reorganized fts_column into tokenizer/, posting/, iterator/ subdirs
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