feat(auto): receipts — record route bindings, rebinds, and sticky vs fresh routing source

## Problem

The ledger cannot answer "which route actually served this session, and why did it change?" `AutoRouteReceipt` (`crates/tui/src/model_routing.rs:503`) records a per-decision receipt, and `RuntimeUsageRecord` (`crates/tui/src/cost_status.rs:821`) records per-route usage, but nothing links turns to a session-scoped route binding or records rebind events. Once sticky bindings exist (#6252), the receipt surface must make them auditable — and the same fields are the training-data labels for any future learned router.

## Change

1. Add `TurnRoutingSource::StickyBinding` beside the existing `ActiveFixedRoute / AutoProviderClassifier / AutoLocalHeuristic` (`crates/tui/src/turn_route_plan.rs:85-93`).
2. Extend `AutoRouteReceipt` and `RuntimeUsageRecord` with: binding id, routing source, and — on rebind — the previous binding id and rebind reason (`credential_lost`, `route_error`, `provider_retired`, `user_override`).
3. Emit a receipt event on binding creation and on every rebind, not only on classifier runs, so the journal (`session_manager.rs:1497-1554`) reconstructs the full binding history of a session.

## Acceptance

- Given a session id, the receipts journal answers: initial binding, every rebind with reason, and per-turn routing source — without consulting live state.
- `RoutedTurnUsage` events carry the binding id end to end.
- Existing receipts keep decoding (additive fields with serde defaults).

Read the routing types in crates/tui/src/turn_route_plan.rs, the receipt and usage records in crates/tui/src/model_routing.rs and crates/tui/src/cost_status.rs, then trace journal emission in session_manager.rs:1497-1554. Done means the journal reconstructs initial bindings, every rebind and reason, per-turn routing sources, and RoutedTurnUsage carries the binding id while existing receipts still decode.

Written by the indexing model from the issue text.