feat(planning): implement constraint-first leave planner

docs/benchmark/README.md

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+# Mobile leave-planner benchmark
+
+Open the deployed application at `/benchmark` on the reference phone. Tap
+**Run benchmark**, wait for the five scenarios to finish, then tap **Copy
+report** and save the tab-separated output with the date and build identifier.
+
+The harness measures preparation, asynchronous calculation, and total wall
+time separately. It records the outcome, last completed quality stage,
+placement count, and determinism between repetitions. `PlanningIncomplete` is
+not a conflict: its last completed stage and incumbent are retained.
+
+## Protocol
+
+- Record phone model, OS version, browser/version, application build, and
+  battery level.
+- Use a cool phone, unplugged, with other applications closed. Record whether
+  the device became warm or throttled during the run.
+- Run the benchmark three times after one warm-up run. Do not interact with
+  the page while a run is active.
+- Representative and loaded scenarios should normally complete below five
+  seconds; every scenario must return within ten seconds. These are release
+  measurements, not CI timing assertions.
+- Keep the copied report unchanged, including any incomplete outcomes or
+  hardware limitations. No mobile-device measurement is claimed by the source
+  tree until a human records it here.
+
+The browser page uses the same `planLeave(request)` operation as the calendar
+and adds no solver or backend dependency.