docs: update for transactions, distributed execution, and protocol changes

[laksamanakeris]

Summary

Docs catch-up covering ~490 commits since the last docs update (d98a6499). All syntax and semantics below were verified against the parser and integration tests before documenting.

• query-language.md — read-your-own-writes transaction semantics (per-transaction staging overlay across all engines), full savepoint syntax and error codes, computed GROUP BY keys, scalar-aggregate identity row on empty input, trailing ENGINE = <name> suffix, RESTORE TENANT ... FORCE + staleness guard, ANALYZE, version() / current_setting(), geometry constructors in INSERT, memory-budget scan bound replacing the silent row cap
• architecture.md — new Distributed Query Execution section (ANALYZE-driven broadcast-vs-shuffle joins, GROUP BY shuffle, grace-hash spill knobs, streaming SELECT), placement reconcile via rendezvous hashing, Raft log compaction gated on the data-plane applied watermark, login rate limits counting only failures
• protocols.md — pgwire driver-compat surface (server_version_num, pg_catalog reflection, per-column binary format negotiation), native-protocol savepoints, HTTP stream in-band error semantics, sync-protocol CollectionSchema announce / typed DeltaReject / idempotent producers / quorum-durable acks
• graph.md — distributed algorithm invocation (same GRAPH ALGO surface), variable-length MATCH expansion caps ([tuning.graph])
• security/auth.md — DROP USER ownership reassignment to {tenant}_admin (fail-closed), login rate limiting section
• bitemporal.md — audit-query _ts_system / _ts_valid_from / _ts_valid_until columns, reserved-column hiding from SELECT *
• databases.md — structural per-engine database_id isolation
• full-text-search.md — per-collection analyzer honored on staged-write (in-transaction) paths
• spatial.md — ST_MakePoint / ST_GeomFromText / ST_GeomFromWKB constructors
• offline-sync-patterns.md — schema announce, constraint rejects with compensation hints, sync durability

Notes

• markdownlint MD013 (line-length) failures are pre-existing across the docs tree and left as-is.