v4.2.0

Waves 1–2 of the 2026-07 performance & stability campaign
(docs/plans/PERF_STABILITY_2026_07/), every item implemented via a reviewed
subagent workflow and gated per docs/GATES.md:

• Extended/prepared protocol unlock — parameterized autocommit reads no
  longer serialize on the session-transaction mutex (atomic fast-out):
  extended point-read 30k-TPS ceiling → 122k @ 64 clients (4.0×); prepared
  34k → 202k @ 32 clients (5.9×) — the PostgreSQL driver-path gap is
  reversed to ~2.4× in Nano's favor.
• Extended-protocol Parse reuse — Describe metadata now derives from the
  shared parameterized plan cache: extended point-read +10–18% on top of the
  unlock (c64 115k → 132k TPS). View DDL (CREATE OR REPLACE / DROP VIEW) now
  invalidates the plan cache (stale-Describe fix).
• COPY fast path for FK/CHECK tables — bulk loads into constrained tables
  no longer fall back to row-at-a-time validation: FK+CHECK COPY 100k rows
  3172 ms → 326 ms (9.7×), with batched index FK probes (including
  batch-local parents), CHECK parity with the slow path, and single-WriteBatch
  all-or-nothing semantics.
• Streaming COPY decode with bounded memory — the wire COPY path decodes
  incrementally across protocol frames (partial-line/quote/UTF-8 state carried)
  instead of buffering the whole stream; new [server] copy_max_buffered_rows
  knob (0 = unlimited) aborts cleanly with zero rows applied when exceeded.
• MVCC bookkeeping diet — prefix-bounded version scans, epoch-micros
  version timestamps (permanent fallback deserializer keeps old on-disk data
  readable), O(1) snapshot-cache invalidation, and unchanged-value
  index-maintenance elision on UPDATE (PG-HOT-style).
• In-transaction read watermark — reads inside a transaction of tables
  unchanged since the snapshot serve from the normal fast read path instead of
  the snapshot scan (30–150×); fail-closed policy (any version-skipping write
  funnel invalidates back to the snapshot path).
• Fix (branch isolation, wrong-data class): ~11 DML sites across both
  execution engines maintained the process-wide value index for branch-routed
  writes — phantom UNIQUE violations on main, and branch DELETEs stripping
  main's index entries for inherited rows. All sites now gate on the same
  predicate that routes branch data.
• Fix: SQL/plan/result caches and the row cache survived USE BRANCH and
  could serve another branch's data; both are invalidated at branch switch.
• Fix (planner): ORDER BY over grouped plans uses the select-list
  aggregate rewrite (was fragile alias-position slicing).
• Fix: TRUNCATE reports no affected-row count (PostgreSQL parity);
  ALTER TABLE bumps the schema generation so stale fast-path caches cannot
  leak backfilled values to open snapshot readers.