Calculate airtime of transmitted and received packets separately

[GUVWAF]

This leverages the new methods getLoRaRxHeaderInfo() and calculateTimeOnAir() introduced in RadioLib 7.3.0 to calculate the airtime of a received packet based on the LoRa header info (coding rate and CRC).
This is mostly important for when going to dynamic coding rates (#8108, #8114), but also now for nodes on older firmware or when people manually set the coding rate.

RadioInterface::getPacketTime() is now pure virtual, hence I had to calculate the preamble time separately (which is now actually more accurate, as it only really considers preamble symbols and not also the LoRa header) and the max. packet length is only calculated when needed (which is very rare). SimRadio uses Meshtastic's original packet length calculation.

[Copilot]

Pull Request Overview

Separates airtime calculation for transmitted vs received LoRa packets, leveraging RadioLib 7.3.0 header introspection for dynamic coding rate and CRC differences. Refactors packet time logic to be virtual, improves preamble timing accuracy, and removes cached max packet time in favor of on-demand calculation.

• Add received-aware getPacketTime(...) API (now pure virtual) across radio interface implementations
• Use RadioLib calculateTimeOnAir with actual received header parameters (coding rate, CRC) for RX airtime
• Refine preamble time calculation to only include actual preamble symbols (exclude header portion)

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/platform/portduino/SimRadio.h	Adds override for new getPacketTime signature.
src/platform/portduino/SimRadio.cpp	Adapts calls to new API; adds local getPacketTime implementation.
src/mesh/SX128xInterface.h	Implements getPacketTime via shared computePacketTime helper.
src/mesh/SX126xInterface.h	Implements getPacketTime via shared computePacketTime helper.
src/mesh/ReliableRouter.cpp	Updates airtime adjustment to use received-aware packet time.
src/mesh/RadioLibInterface.h	Introduces computePacketTime templated helper using RadioLib header info.
src/mesh/RadioLibInterface.cpp	Updates receive detection logic and RX interrupt handling to use new timing.
src/mesh/RadioInterface.h	Makes getPacketTime(uint32_t, ...) pure virtual; updates overloads.
src/mesh/RadioInterface.cpp	Removes old generic time-on-air calculation; updates preamble timing computation.
src/mesh/RF95Interface.h	Implements getPacketTime via computePacketTime.
src/mesh/LR11x0Interface.h	Implements getPacketTime via computePacketTime.

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