Report MTU scan progress while the scan runs

[TaJirax]

What changed

• adds logMTUProgress, which reports completed, total, valid, and rejected as WD_PROGRESS phase=mtu across the 10-80 percent band
• throttles the output to one line per percent step, never dropping the first or last line of a scan
• calls it from a single defer in runConnectionMTUTest, registered ahead of the panic recovery so every exit path reports after the counters have been updated

Why

The MTU scan is the longest phase of a connect, and it emitted no WD_PROGRESS at all. A client watching the machine output saw the bar sit at the starting percent and then jump to selecting when the scan finished, with no resolver counts in between. The WhiteDNS desktop app draws its connection progress bar and its scan resolver counts from exactly these fields, so both were frozen for the whole scan.

Validation

• new tests cover emission at LOG_LEVEL=WARN, where the human-readable per-resolver lines are suppressed, and the throttle behaviour including the final line
• go build ./..., go vet ./..., and go test ./... pass

🤖 Generated with Claude Code

[TaJirax]

Superseded by #4, which merged this commit as part of the full engine sync. The MTU scan progress reporting is on main.