Cooling-stage clock sweep (batched sweep + HOT/COOL evictor + strategy removal)

[gburd]

Cooling-stage clock sweep for the buffer manager — CI check.

This PR is against gburd/postgres:master (my fork), not upstream, purely
to exercise CI on the three-patch series before posting to pgsql-hackers.

1. Batch the clock sweep to reduce nextVictimBuffer atomic contention
2. Replace the usage_count clock sweep with a cooling-stage evictor
3. Remove BufferAccessStrategy; scan resistance is now intrinsic

Net +510/-1642 over 83 files (almost all deletion is patch 3).

Each commit builds -Werror + cassert + injection_points and passes
regress/regress (245 tests) independently; verified on both the fork base
and a clean upstream/master rebase. See the draft cover letter for the
design reasoning and the m6i / r8i (6-node NUMA) / huge-pages / local-NVMe
real-IO benchmark data.

Not for merge — this is the review/CI vehicle for the -hackers submission.