v0.9.4

Closes the per-engine gap between MySQL / SQL Server and the PostgreSQL
reference on the axes a DBA actually feels — keyset coverage, pooler-safe
session state, and the adaptive governor's pressure signal — driven measure-
first, so the most expensive idea (a MySQL server-side cursor) was disproven
with evidence before any code was forked. No public-API breaks.

Added

• feat(mssql) — keyset (seek) pagination on any single-column unique NOT
  NULL index, not just the PK (parity with PostgreSQL/MySQL, which already
  did). Tables with a surrogate/business unique key (and a composite or non-int
  PK) now take the safer keyset path instead of falling back to the less-safe
  chunked path, and chunk_by_key: <unique non-PK col> is accepted.

Fixed

• fix(mysql) / fix(mssql) — pooler-safe session teardown. Both drivers
  now reset the per-connection session state they mutate (time_zone +
  max_execution_time on MySQL; LOCK_TIMEOUT on SQL Server) on a Drop path,
  so a connection returned to the pool — or reused behind ProxySQL / MaxScale /
  an Azure SQL gateway — can't carry our settings into the next checkout, even
  on a panic or early return. MySQL gains a MysqlSessionGuard (analogue of
  PgTxnGuard); SQL Server gains a Drop reset (best-effort, time-boxed). This
  also closes a MariaDB-specific leak (it spells the timeout max_statement_time,
  so the old end-of-function reset's ? could skip the time_zone reset).

Changed

• feat(governor) — MySQL/SQL Server adaptive governor now samples
  extraction pressure, not write pressure. The old Innodb_log_waits /
  Log Flush Waits signals are redo/log write pressure that barely moves
  during a read-only export. Replaced with the PostgreSQL temp_bytes analogue:
  MySQL sums Created_tmp_disk_tables + Innodb_buffer_pool_wait_free (sort
  spill + buffer-pool memory pressure); SQL Server sums cumulative
  Workfiles Created + Worktables Created (sort/hash spill to tempdb). Both
  monotonic, so the governor's cur > prev contract is unchanged.
• docs(tuning) — chunk_size documented as the statement-duration lever.
  Lowering chunk_size is the supported, measured way to keep each MySQL/SQL
  Server read short under a strict statement_timeout (one statement per chunk),
  with the trade-off (more files, ~25% throughput, not a speed win) stated
  honestly. A MySQL server-side cursor was investigated and rejected with a
  libmysqlclient measurement (dev/spikes/mysql_cursor_efficacy.c): its
  read-only cursor materialises the result into temp tables at open, so it would
  be a regression vs lowering chunk_size, not an improvement.