docs(benchmark): record PR #278 benchmark headroom

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/benchmark/README.md

@@ -16,8 +16,14 @@ handoff for #78 are recorded in [the #77 verification report](../trace-free-plan
 The benchmark is required again in Gitea Actions, including the image and
 aggregate gates. [The #78 report](issue-78.md) records the baseline timeout,
 profiling, before/after diagnostics, full protocol output, correctness checks
-and browser limitations. The local pass has only 1.4 seconds of external-timeout
-headroom; CI-runner performance and live workflow validation remain unverified.
+and browser limitations. That pass had only 1.4 seconds of external-timeout
+headroom. Three later exact optimizations for PR #278 cut the Vitest duration
+to **228.1 seconds** (71.9 seconds of headroom) in the most recent local run:
+[direct date keys](pr-278-date-keys.md),
+[immutable date counts](pr-278-immutable-date-counts.md) and
+[mandatory-move prechecks](pr-278-mandatory-moves.md). Outcomes, explanations
+and final states are unchanged. CI-runner performance and live workflow
+validation remain unverified.
 The existing migration protection stays in place under
 [issue #268](https://gitea.nessar.fr/nessar/vac-optimizer/issues/268).