Reduce monitor noise and classify canceled downloads

[pzzzy]

What changed

• Let the loopback FTP monitor identify itself with CLNT macftpd-monitor.
• Retain one completed monitor cycle per hour, suppress intermediate successful probe actions, and always retain probe failures.
• Classify broken pipes, resets, context cancellation, and canceled HTTP/2 streams as canceled downloads rather than server failures.
• Separate client cancellations from failures in the weekly report and document how to interpret them.
• Add focused tests and extend the protocol lab to require CLNT support.

Why

The live health review found that the service was stable, but approximately 10,000 successful monitor actions over seven days were crowding human activity out of the bounded activity store. The same review found client disconnects and tunnel cancellations recorded as download failures, obscuring genuine server faults.

Impact

The activity dashboard and weekly report retain longer, higher-signal history. Completed downloads and server-side failures keep their existing semantics; clients that stop reading are now visible as a separate operational category.

Validation

• ./scripts/check.sh
• targeted FTP, HTTP, and activity tests
• synthetic weekly-report fixture confirming cancellations and failures render in separate sections