feat(host): own session event delivery with bounded mailboxes

[noho]

Summary

• make Host the sole owner of Session Event Delivery with an async attach boundary
• add one item-bounded mailbox per subscription, counted in-flight retention, per-Session admission, typed errors, and packaged defaults of 512 items and 4 subscriptions
• add durable causal fencing, bounded catch-up, mailbox-empty reconciliation, cross-opener ordering barriers, and local terminal post-commit coordination
• remove the Service event-copy relay and use one exact-five observation consumer with deterministic cleanup
• isolate CLI display callbacks on a private single-worker executor and preserve prompt/interactive terminal and cancellation semantics
• wire runtime config, assembly, public callers, tests, metrics, and README audits

Contract boundaries

• no byte or resident-heap bound
• no delta persistence, replay, or reconnect backfill
• no third sequence domain or durable/transient global order
• no cross-process terminal broadcast
• no Host-global cross-Session quota
• no WU-CLI-SMOKE-01-R2 changes
• no GitHub Issue is associated with this user-adjudicated work unit

Validation

• affected Host, runtime, Service, and CLI suites: 3443 passed, 9 skipped, 6 deselected
• stress tests: 6 passed
• full pyright: 0 errors
• changed core production files: 84% to 95% single-file coverage
• aggregate deepreview: independent AgentMiMo and AgentDS PASS, 0 material findings
• git diff check and stale ownership scans passed