fix(undo): reset transaction-local segment heads on commit

[sqlrush]

What this changes

Resets transaction-local undo head and touched-segment bookkeeping after a normal commit while preserving the backend-local residual extent and file-descriptor cache. Adds a persistent-backend TAP regression that crosses the 1024-entry boundary.

Pre-fix evidence: 1024 updates committed and the 1025th failed with SQLSTATE 53R9D. The fixed binary completes all 1030 updates.

How it was tested

• macOS enable-cluster debug/cassert build and install
• cluster_unit: 177/177 binaries passed
• cluster_regress: 13/13 passed
• targeted t/270: 31/31 passed
• PostgreSQL regression: 219/219 passed
• release enable-cluster build without cassert/debug: passed
• disable-cluster build/install: passed; PostgreSQL regression 219/219; no cluster entry-point symbols
• comment headers, clang-format (568 files), SCN/GES/CLOG/release-note policy checks, and git diff --check: passed

Checklist

• The --disable-cluster build stays binary-equivalent to upstream PostgreSQL 16.13 and passes the 219-test regression suite.
• Required runtime safety has an explicit production branch; Assert is defense-in-depth only.
• Code matches repository formatting rules.
• Tests updated.