feat: reporting-allocate AI-plan cost across work

Split a booked AI-plan fact across workplans or repos using measured
tokens, measured+estimated tokens, session counts, or an even split.
Unmeasured remainder stays a residual. The record is not resource-control
AllocationEvidence and does not create booked spend.

tests/fixtures/session-tokens-two-workplans.json

@@ -0,0 +1,47 @@
+{
+  "schema_version": "0.1",
+  "record_type": "session_token_evidence",
+  "record_id": "session-tokens:state-hub:2026-08:two-workplans",
+  "revision_of": null,
+  "source_system": "state-hub",
+  "period_start": "2026-08-01",
+  "period_end": "2026-08-31",
+  "captured_at": "2026-08-15T12:00:00Z",
+  "source_evidence": [
+    "state-hub:/token-events/aggregate/?month=2026-08"
+  ],
+  "totals": {
+    "measured": {
+      "tokens_in": 200,
+      "tokens_out": 0,
+      "event_count": 3,
+      "confidence": "1.0"
+    },
+    "estimated": {
+      "tokens_in": 50,
+      "tokens_out": 0,
+      "event_count": 1,
+      "confidence": "0.35"
+    }
+  },
+  "associations": [
+    {
+      "scope": "workplan",
+      "scope_id": "workplan-a",
+      "label": "WP-A",
+      "measurement_kind": "measured",
+      "tokens_in": 100,
+      "tokens_out": 0,
+      "event_count": 1
+    },
+    {
+      "scope": "workplan",
+      "scope_id": "workplan-b",
+      "label": "WP-B",
+      "measurement_kind": "measured",
+      "tokens_in": 50,
+      "tokens_out": 0,
+      "event_count": 1
+    }
+  ]
+}