perf(planning): retain the best valid plan more cheaply

Retaining a better valid plan reconciled its optimization provenance,
which evaluated every active objective once per credited placement with
that placement released. Only the credited objectives are compared, so
the released plan now evaluates just those and skips observing the
others. Retention also stops computing quality metrics that nothing
read. The retained plan and what Cancel returns are unchanged.

Part of #302.

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

docs/benchmark/issue-302.md

@@ -35,3 +35,27 @@ the placements in the same order with the same yields.
 | loaded year                 |      58.26 |     57.75 |
 | two years                   |       2.27 |      1.82 |
 | three years                 |       7.45 |      5.52 |
+
+### Retain the best valid plan more cheaply
+
+Retaining a better valid plan for cancellation reconciled its optimization
+provenance. For every placement credited by an optimization rule, that
+evaluated all active objectives with the placement released. Only the credited
+objectives are compared, so the released plan now evaluates just those, and
+the calendar observations of the other objectives are no longer computed.
+Retention also no longer computes quality metrics that nothing read. An
+instrumented three-year run spent 1.16 of its 1.22 retention seconds in that
+reconciliation; afterwards retention took 0.26 seconds.
+
+| Scenario                    | Before (s) | After (s) |
+| --------------------------- | ---------: | --------: |
+| representative year         |       0.45 |      0.37 |
+| typed obligations           |       4.76 |      5.03 |
+| chained earned entitlements |       0.02 |      0.02 |
+| deliberate conflict         |       0.01 |      0.01 |
+| loaded year                 |      55.59 |     53.58 |
+| two years                   |       1.68 |      1.22 |
+| three years                 |       5.29 |      3.85 |
+
+The typed-obligations difference is run-to-run noise; its retention cost was
+22 milliseconds.