Cascade measurement: routing telemetry, session summary, aggregate report #1136

Parent: #1134

Measurement from day one — the mode must prove itself:

• Telemetry events (existing pipeline): route_primary_attempt, route_escalated (with gate reason), route_completed (lane, wall-clock, token counts per lane).
• Per-session summary line at exit: what ran where, gate outcomes, estimated cost delta vs escalation-only, added latency when escalation happened.
• Aggregate report (altimate route report or equivalent): escalation rate by task class, gate-check failure distribution, primary success rate trend over time, cost/time totals.
• Privacy: task classes are coarse categories, never prompt contents.
• Tests: event emission, summary math (cost/latency accounting), report aggregation.