#29600 base red: CmuxNextAppTests compile again (missing CmuxNextAgentPane import, slow #expect)

Summary

Base red, priority zero (tip watcher STATUS 2342). The feat tip 8769d23ba27 fails the CmuxNext compile check, job e0fc122468454277f36ec6eb, in two test files:

• CloudMachineChangeStateTests.swift (from #28407) uses DaemonUpdateBoard, which is in CmuxNextAgentPane, without import CmuxNextAgentPane. The .paused error at :548 follows from it.
• AddMachineFlowTests.swift:189 (from #28762): list.rows.map(\.step).filter { $0 == .approvals }.count == 1 inside #expect is too slow for the type checker. Filter rows by step first, then count.

Test-only. No product code changes.

Testing

• Red: the tip's own compile check e0fc122468454277f36ec6eb at 8769d23ba27 (7 errors, all in these two files).
• Green: fleet compile check at this head (job in a comment below).
• prepush-gate.py: no new violations; arch-ratchet: no count rises.

Observed / Not run

• Observed: the tip's error list (tip watcher state), the fix diff, prepush-gate.
• Not run: the two suites themselves; this PR only makes them compile.

Changelog

none

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Summary by cubic

Fixes the red CmuxNext compile check on feat-cmux-next caused by two test files that failed to compile.

• Adds the missing import CmuxNextAgentPane in CloudMachineChangeStateTests, which uses DaemonUpdateBoard from that module.
• Rewrites the slow map/filter/count chain in AddMachineFlowTests inside #expect to filter rows by step first, then count.

Test-only; no product code changes.