ci: the macOS plain leg skips four platform-independent gates, and main runs are no longer cancelled by the next push

Main's first full sharded run (34155361413) was green at 37 min with the critical path on the macOS PLAIN leg
(33-37 min per shard; macOS Release 25-26, every Linux shard ≤ 23): an -O0 binary on a 3-core runner, and
GitHub's five-concurrent-macOS cap rules out more shards. Its four slowest gates — binoverridecheck,
knownitemcheck, ripwirepubliccheck, xmlwellformed (545/391/383/349 s there) — assert nothing platform-specific
and still run on the macOS Release leg and all four Linux legs. test/pargates.py gains --exclude-list FILE (a
committed file, one gate and its reason per line, applied before the shard split, a stale name is an error);
the macOS plain matrix rows point at .github/pargates-macos-plain-skip.txt. Separately, cancel-in-progress
now applies to pull requests only: three consecutive main runs today were cancelled by unrelated pushes with
zero failures among the 45 jobs that had completed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>