docs: update README with all missing features and API documentation (closes #65)

[s2x]

Description

Closes #65

Complete rewrite and expansion of the README with all missing API documentation sections. The README now covers every public method across all classes and includes comprehensive reference tables for data types, index types, metrics, and quantization.

Changes

Added new sections

• ZVecIndexParams — all 6 factory methods (forHnsw, forHnswRabitq, forFlat, forIvf, forVamana, forInvert)
• ZVecCollectionOptions — constructor, readOnly/readWrite/defaults factory methods
• ZVecFieldSchema — all 12 getter methods for field introspection
• ZVecCollectionStats — doc count, index count, completeness APIs
• ZVecVectorQuery — all 12 fluent setters + fromId()
• ZVecGroupByVectorQuery — constructor, setters, unsupported methods doc
• ZVecReRanker (Interface) — interface definition
• ZVecRrfReRanker — RRF formula, getters/setters
• ZVecWeightedReRanker — weighted fusion, all getters/setters
• ZVecRerankedDoc — combined score, source ranks/scores
• Data Types — complete 28-row table with constants, values, PHP types
• Index Types — 6 index type constants with descriptions
• Metric Types — 4 metric constants
• Quantize Types — 5 quantize constants

Updated existing sections

• ZVec (Collection) — lifecycle (init/isInitialized/shutdown/version API), schema operations (8 addColumn* + alterColumn), index management (createIndex with ZVecIndexParams), batch operations (insertBatch/upsertBatch/updateBatch), all search variants (queryVector/queryFp16/queryFp64/queryMulti/queryByFilter/queryById/queryWithReranker/groupByVectorQuery)
• ZVecSchema — all 24+ add* methods (scalar, dense vector, sparse vector, binary, array types + deprecated aliases)
• ZVecDoc — all setters, getters, introspection, serialization, operator tracking, memory usage
• Known Limitations — added alterColumn(), GroupByVectorQuery, queryById() constraints

Stats

• README expanded from 370 → 891 lines (+521 lines)
• All PHP code examples verified syntactically valid

Testing

• All .phpt tests pass (pre-existing failures from native extension loading are unrelated)
• No test database leftovers
• Quick Start code example verified syntactically valid PHP

Code Review

• Passed subagent code review
• All review comments addressed