perf(pr-pipeline): raise merge.max-in-flight from 5 to 10

.changes/pr-pipeline/perf-max-in-flight-10-5b1c7e2a.md

@@ -0,0 +1,7 @@
+---
+bump: patch
+---
+
+### Changed
+
+- This repository's `merge.max-in-flight` rises from 5 to 10. One full run of main's tip verifies every lane merge it contains, so the lane arms up to twice as many pull requests per verification cycle with no extra full runs. A red run now charges up to 10 unverified lane merges, and any charge still demotes the rung.

docs/conventions/pr-pipeline.yaml

@@ -970,7 +970,7 @@ merge:
   # Lane merges in flight at once (queued plus merged and not yet verified).
   # The merge queue tests each PR against the latest base plus the PRs ahead
   # of it.
-  max-in-flight: 5
+  max-in-flight: 10
   # Paths the PR checks read (git pathspecs). When main changed one since a
   # PR's merge base, the gate updates the PR from main instead of enqueuing
   # it, and a later run enqueues the new head once its checks pass: a head