docs(adr): ADR-001 — reactive state via scoped StateBus and ownership-based discovery

[WilliamKarolDiCioccio]

Summary

• Replaces five per-PR design docs (state-bus.md, agent-session-agent-state.md, agent-runtime-thread-state.md, agent-session-bus-route.md, agent-state-as-bus-view.md) with a single consolidated ADR covering the full PR feat(soliplex_client): GenUI P0 — Surface + StateProjection + StateBus #189–refactor(soliplex_agent): agentState is now a view of bus.agentState #193 stack
• Documents the key design decisions that emerged across the stack: ThreadKey over bare threadId, @immutable ThreadState with withHistory, ensureThreadState vs threadStateOf, runtime-owned disposal cascade, and ownership-based discovery as the RAII substitute
• Flags the late final agentState / plain bus getter asymmetry as a contractual (not mechanical) safety guarantee — the invariant that _threadStates[key] is never overwritten with a fresh ThreadState for an existing key must hold for agentState to stay on the correct bus

What changed from the per-PR docs

The five source docs were written incrementally as each PR landed. The ADR reflects the final state after #193 collapsed the parallel computed() path into bus.agentState. Key corrections vs the original per-PR framing:

• "Host (per-thread)" replaced by AgentRuntime as the concrete bus owner throughout
• agentState data-flow diagram updated to the PR refactor(soliplex_agent): agentState is now a view of bus.agentState #193 final form (_onStateChange → bus.setAgentState → bus.agentState = session.agentState)
• Exhaustive sealed-switch argument repositioned: the switch feeds the bus via _onStateChange, it no longer powers agentState directly
• Lifecycle sequence diagram shows the Runtime → ThreadState → StateBus cascade, not a manual host teardown

Test plan

• markdownlint-cli2 — clean
• No code changes; docs only

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