test: refresh PostgreSQL coverage for definitions and session commands

Regenerate the pinned PostgreSQL 18.4 correctness snapshot after f7d8af4,
including per-statement results, deterministic CI fixtures and the detailed
compatibility backlog. The upstream corpus and extraction policy are unchanged:
51,415 distinct statements retain their source file/line provenance.

Deep support increases from 45,810 to 48,313 statements (89.1% to 94.0%).
The 2,503 improvements consist of 1,685 prior errors, 551 statement-type
mismatches, 266 trailing-input results and one partial parse. No previously
deeply supported case is lost. The remaining inventory contains 2,302 errors,
458 trailing-input results, 290 type mismatches and 52 partial parses.

Compare every refreshed row with the independently reviewed replay, including
statement types, completion status, AST presence and unconsumed input. All
2,503 new deep cases also retain equivalent PostgreSQL raw ASTs after SQL
emission, removing source-position fields only. The generated suite now has
6,353 CI cases; reductions reflect resolved backlog entries rather than
removal of source statements.

Validation: the full pinned-corpus compatibility command passes against the
refreshed baseline and generated CI fixtures. The implementation was also
validated by 1,472 passing tests (37 external-backend skips), 17 ASan/UBSan
tests and 87 compatibility-harness tests. Record completion in the plan.

This is a correctness corpus, not a performance benchmark. Benchmark files
and benchmark-only documentation/configuration edits remain uncommitted.

Author: renecannao