feat(observability): per-voter, per-decision cost aggregation on consensus + pr_review

[williamzujkowski]

Part of #3854.

Context

Each panel run fans out N voter calls; token usage is visible per call (adapter layer) and pricing per model is in the registry. Aggregate: decisionId → per-voter {model, tokens, cost} → decision total; persist alongside the existing vote/outcome records (record + measure; no routing change).

Acceptance criteria

• consensus_vote + pr_review runs persist a per-decision cost rollup (per-voter breakdown)
• Pure rollup function fixture-tested (the observability: durable policy audit (#3710) records violations only — no allow-baseline for the would-block rate #3727 report pattern); plan-mode (NEXUS_BILLING_MODE=plan) recorded as 0-cost but token counts kept
• No new MCP tool; surfaces via existing records

Evidence required

Fixture tests + one live-run sample (BLOCKED note if no live adapter at the time).

Out of scope

weather_report/manifest surfacing (next child).