Respect dynamic CR in transmit timing

[robekl]

This patch makes transmit timing respect dynamic coding rate (CR) instead of assuming the node's default CR.

What Changed

• Added a shared estimateTxAirtimeFor(...) hook in Dispatcher so timing paths can use a per-packet CR when one is selected.
• Updated retransmit delay and send-expiry calculations to use _tx_cr.
• Updated direct-send timeout estimation to set _tx_cr before estimating airtime, so sender-side timing reflects the selected first-hop dynamic CR.
• Added exact LoRa airtime overrides in the mesh implementations that know their active SF/BW/CR (companion, repeater, room_server, sensor, and simple_secure_chat).
• Synchronized setDefaultCR(...) anywhere runtime radio parameters are applied or restored.

Why This Satisfies the Goal

The goal was to update retransmit delay and transmit-duration calculations based on dynamic CR.

This patch does that by making:

• retransmit delays use the effective per-packet CR
• dispatcher transmit-expiry timing use the effective per-packet CR
• direct-send timeout estimates use the selected first-hop CR when that CR is actually knowable

Notes

• Airtime estimation does not mutate live radio state.
• Sender-side multi-hop timeout estimation is only exact for the local hop, because downstream repeater CR choices are not knowable in advance.

[weebl2000]

Thank you, I will have a look at this over the weekend most likely!