app: bus manager owns the adapter with lanes, spans, coalesced writes and a debug readout

[aq1018]

PR 1 of three from docs/design/bus-manager.md. One scheduler owns the client: exclusive, control, refresh and live lanes under the document's decision rule; subscriptions merged into spans per servo and rate class; decode once and fan out one immutable Snapshot; writes coalesce per (servo, register) and dirty the covering span; utilisation-based stretching; group reads behind a gread feature detect; a register cache that serves a subscriber mounting mid-period without an exchange. The React binding publishes once per animation frame. The session hosts the manager, runs scan and baud migration as one exclusive turn, and the fleet cards are one slow subscription per servo. ?debug renders the exchange statistics.

• 26 manager unit tests over a manual clock and a fake client, plus store and span tests
• tests/e2e/bus.spec.ts drives the debug readout over the simulated fleet
• card-poll and command-queue deleted; Session.run and Session.client stay until PR 3

https://claude.ai/code/session_015YgEehU5GLftenbuhpKWMi