Merge branch 'kk/merge-base-exhaustion' into seen

Merge branch 'kk/merge-base-exhaustion' into seen

The merge-base computation has been optimized by stopping the walk
early when one side's exclusive commits in the queue are exhausted,
yielding significant speedups for queries with one-sided histories.

* kk/merge-base-exhaustion:
  commit-reach: remove commit-date ordering fallback
  commit-reach: move min_generation check into paint_queue_get()
  commit-reach: terminate merge-base walk when one paint side is exhausted
  commit-reach: introduce struct paint_state with per-side counters
  t6600: add clock-skew topologies and step counts for edge cases
  commit-reach: add trace2 instrumentation to paint_down_to_common()
  t6099, t6600: add side-exhaustion regression tests
  t6600: add test cases for side-exhaustion edge cases
  test-lib-functions: improve diagnostic output for trace2 data assertions
  Documentation/technical: add paint-down-to-common doc