#21422 feat(cmux-next): wire ACPmux events into priority agent view

What changed

Wire the existing native ACPmux _acpmux/watch lane feed into the Agent Activity priority view without adding polling or a second session identity model.

• Preserve the existing sessionId -> workspaceId binding on AgentActivityLane and carry it through the web bridge.
• Track a monotonic priority revision and changed session so only the hook-updated row gets the transform-only bounce.
• Feed priority ordering from the existing event-driven AgentTurnStateFeed / AgentLaneStore path.
• Keep the web event reducer defensive for duplicate and out-of-order ACPmux session events.

Validation

• bun test ./test/agent-activity-priority.test.ts ./test/agent-activity-lanes.test.tsx — 12 passed.
• git diff --check — passed.
• swiftc -parse Packages/macOS/CmuxNext/Sources/CmuxNextAgentActivity/AgentActivityLane.swift Packages/macOS/CmuxNext/Sources/CmuxNextAgentActivity/AgentActivityModel.swift Packages/macOS/CmuxNext/Sources/CmuxNextAgentActivity/AgentActivityHostView.swift — passed.

Full native build was not run on the M5 MacBook Air per repository build policy.

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Summary by cubic

Wires the native ACPmux _acpmux/watch lane feed into the Agent Activity priority view without adding polling or a second session identity model.

• Carries the existing sessionId → workspaceId binding through the web bridge so priority rows jump to the correct workspace, and retains a session's binding when a later hook event omits it.
• Tracks a monotonic priority revision and changed-session marker so only the hook-updated row gets a transform-only bounce animation.
• Validates each ACPmux session summary at runtime and keeps the reducer defensive against duplicate, out-of-order, and purged events.