perf(planning): copy only the debited balance when assigning a date

Assigning a date copied every balance and every earned activation.
Balances and activations are only edited right after a state copies
them, so the new state now shares them with its parent, except the
balance it debits. Placement copying is unchanged.

Part of #302.

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

docs/benchmark/issue-302.md

@@ -108,3 +108,26 @@ the sum. The core therefore scores them without revalidating;
 | loaded year                 |      48.12 |     44.03 |
 | two years                   |       1.04 |      0.96 |
 | three years                 |       3.35 |      3.03 |
+
+### Copy only the debited balance when assigning a date
+
+Assigning a date copied every balance and every earned activation. Balances and
+activations are only edited right after a state copies them, so the new state
+now shares them with its parent, except the balance it debits. Placement copying
+is unchanged. The benchmark scenarios have at most six balances and few earned
+activations, so the gain is within run-to-run noise.
+
+| Scenario                    | Before (s) | After (s) |
+| --------------------------- | ---------: | --------: |
+| representative year         |       0.29 |      0.28 |
+| typed obligations           |       3.38 |      3.27 |
+| chained earned entitlements |       0.01 |      0.01 |
+| deliberate conflict         |       0.00 |      0.00 |
+| loaded year                 |      43.93 |     44.05 |
+| two years                   |       0.95 |      0.92 |
+| three years                 |       3.00 |      2.97 |
+
+Across the five exact steps, the same harness measured three years at
+7.45 seconds before the first step and 2.97 seconds after the last, two years at
+2.27 and 0.92 seconds, typed obligations at 5.11 and 3.27 seconds, and the loaded
+year at 58.26 and 44.05 seconds. These figures come from different sessions.