MeshCore good-citizen controls: airtime governor + budget policy

[M0LTE]

Parent: #137. Model A (ride the public preset, unscoped) floods the whole same-preset network, so DAPPS must self-limit hard.

• Port the PoC TxBudget airtime governor into the bearer data path — a hard gate (back-pressure, not drop). Default 30 s/hr ≈ 0.83 % duty (12× under the 10 % cap) — decide the budget policy.
• Size discipline: cap packets/message on the public preset; route bulk to Models B/C.
• Heavy compression (separate issue) compounds with the governor (1 pkt/msg is what makes the rate viable).
• Observability of airtime/duty + the master TX kill-switch at the bearer chokepoint.

Demonstrated in the PoC (budget-test, and live send accounting). See README "Good-citizen controls".