test: refresh PostgreSQL corpus after native grammar continuation

Regenerate the pinned PostgreSQL 18.4 correctness baseline, selected CI
fixtures and compatibility report against implementation commit c3e1714.
The corpus remains the same 51,415 distinct oracle-accepted statements
from the official PostgreSQL source distribution; source provenance and
upstream parser pins are preserved.

Deep support increases from 43,373 to 45,810 statements (84.4% to 89.1%):
2,437 additional statements with no previously supported case lost.
The final distribution is 45,810 DEEP_SUPPORTED, 3,987 ERROR,
724 TRAILING_INPUT, 841 TYPE_MISMATCH and 53 PARTIAL; no cases are
classification-only. The generated CI selection now contains 8,681 cases.

Review status changes among already unsupported statements explicitly:
EXPLAIN now propagates inner failures/incomplete syntax, unsupported
COLLATION FOR expressions retain incomplete results, and a repeated
constraint-enforcement clause leaves visible trailing input. No status
change in this group removes previously complete native grammar support.

Verify every regenerated row against the separately reviewed full replay,
including statement classification, AST presence, result and remaining SQL.
Run the full compatibility workflow against the refreshed baseline and
selected CI fixtures, in addition to 87 passing Python harness tests.
The implementation's full C++ suite and focused sanitizer results are
recorded in the preceding commit and PR description.

This is a correctness corpus refresh, not a performance benchmark. No
benchmark scripts, timings, binaries or benchmark-only documentation are
included. Remaining PostgreSQL grammar gaps remain enumerated in the report.

Author: renecannao