IB-WP-0019-T04: plan-vs-actual variance and surfacing

After every generate run, compute variance between the executing plan
snapshot and the just-recorded usage rollup, persist it to
output/budget/summary.yaml (overwrite-on-run), and surface it both in
the generate status JSON (new budget_summary field) and as a "Plan
variance" line in reports/generation-summary.md.

Variance fields: calls / prompt_tokens / total_tokens each carry
{estimated, actual, delta, ratio}; cost_usd carries {estimated,
actual_known, actual_estimated_from_rates, actual_total, delta, ratio};
per_workflow rolls the per-bucket usage up to the same workflow_id grain
the plan reports. Runs whose snapshot_id cannot be resolved (no prior
plan, or pruned from the retention window) still record a variance row
with null comparison fields and snapshot_resolved=false, so the
consumer always sees a current summary.

Reordered run_generation so usage and variance are written before the
generation report, allowing the report to embed the variance line on
the same pass.

110 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workplans/IB-WP-0019-budget-and-usage-registry.md

@@ -137,7 +137,7 @@ state_hub_task_id: "688c590d-8885-455e-bcf6-61409a45e001"
 
 ```task
 id: IB-WP-0019-T04
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "c6adc4fb-9062-4c81-a0b2-98d3166e047d"
 ```