nodes.stats: each node's airtime and duty cycle

[A13xB0]

The engine counts each node's time on the air and the Scores panel draws it, but a script could only get the whole mesh's total, out of an experiment cell. "Which repeater is over its duty cycle" had an answer on screen and none outside it.

Each nodes.stats row now carries airtime_ms (cumulative over the run, so a window is two readings subtracted) and duty_cycle_pct (over the run's whole length, as the panel's column is). A node the engine has no score for has neither rather than a zero. The Python client's NodeStat has both fields.

In the GUI: the same numbers are the Scores panel's airtime and duty columns, unchanged; this makes them reachable from a script.

The Scores panel, where these numbers already were:

The same run's nodes.stats rows now carry them:

{'name': 'CaltonHill', 'sent': 3, 'airtime_ms': 3591, 'duty_cycle_pct': 19.3}
{'name': 'alba', 'sent': 3, 'airtime_ms': 3294, 'duty_cycle_pct': 17.7}
{'name': 'bravo', 'sent': 0, 'airtime_ms': 0, 'duty_cycle_pct': 0}

Tests: rows matched by name, and a node with no score carries no airtime.

Docs: MeshBench/docs#44