Route per-board logic through Board.get_type()

[SaintSampo]

Summary

Replaces the scattered board-name string checks ("NanoXRP" in implementation._machine, "Beta" in ...) with a single detection point on Board, so per-board logic reads as data/identity comparisons instead of duplicated _machine matches — and adding a future board is a one-line change in get_type() rather than edits across the library.

Changes

board.py — the one detection point

• Adds Board.XRP / Board.BETA / Board.NANO constants and a cached Board.get_type(). This is now the only place a machine name is matched.
• Its own are_motors_powered / get_battery_voltage checks use it.

Call sites converted to Board.get_type() == Board.<X>

• differential_drive.py — wheel diameter/track and nominal_voltage (ternaries); the _move tuning block keeps its if on the clean identity.
• encoded_motor.py — motor class (SinglePWMMotor/DualPWMMotor) as a ternary; the default speed controller keeps its if/else with one PID param per line.
• motor.py — flip_dir ternary.
• imu.py — the cached _is_nanoxrp init flag; the hard-IRQ update handler still reads that cached bool, so get_type() is never called from an interrupt.
• encoder.py — the resolution gear-ratio if/else.

Also includes a small related fix (separate commit): dashboard.py reads battery voltage via Board.get_battery_voltage() instead of a raw ADC, so it uses the per-board formula and, on the Nano, restores the shared VIN/RM2 pin after reading.