perf(planning): reuse the donor ranking across public-holiday weeks

The holiday-week stage ranked every placement as a donor once per
public-holiday week, although the ranking depends only on the plan. The
weeks of one plan now share each placement's released objectives; each
week still skips its own dates and walks the placements in the same
order with the same yields.

Part of #302.

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

docs/benchmark/issue-302.md

@@ -0,0 +1,37 @@
+# Issue #302: plan from the best-ranked obligation solution
+
+Step-by-step record for [issue #302](https://gitea.nessar.fr/nessar/vac-optimizer/issues/302).
+The exact steps keep planning outcomes, stable explanations and final states
+identical to the frozen engine at `a871f8f`. The
+[frozen-engine differential check](README.md#frozen-engine-differential-check),
+including its cancelled asynchronous comparisons, passes after every step.
+
+## Per-step measurements
+
+Each step was timed against its parent in the same session, on the development
+machine (AMD Ryzen 7 7800X3D, WSL2, Bun 1.4.3), with no other test, build or
+profiling workload running. Both commits' cores were bundled with the
+frozen-engine build command and called through `calculatePlanningAsyncInternal`
+in one Bun process, with `calculationDate: '2024-01-01'`. This is the
+benchmark's in-thread asynchronous path, without the planning-calculation
+service. For every scenario, each engine ran one warm-up and then one measured
+calculation, alternating engines. Outcomes, explanations and final states were
+hashed; every pair below matched. Only same-session pairs are comparable.
+
+### Reuse the donor ranking across public-holiday weeks
+
+For every public-holiday week, the holiday-week stage ranked every placement as
+a potential donor by releasing it and evaluating the active objectives. That
+ranking depends only on the plan, so the weeks of one plan now share each
+placement's released objectives. Each week still skips its own dates and walks
+the placements in the same order with the same yields.
+
+| Scenario                    | Before (s) | After (s) |
+| --------------------------- | ---------: | --------: |
+| representative year         |       0.66 |      0.66 |
+| typed obligations           |       5.11 |      4.89 |
+| chained earned entitlements |       0.02 |      0.02 |
+| deliberate conflict         |       0.01 |      0.01 |
+| loaded year                 |      58.26 |     57.75 |
+| two years                   |       2.27 |      1.82 |
+| three years                 |       7.45 |      5.52 |