Safe POM CV bit read/write

[jaak0b]

Adds safe (retrying, deadline-bounded) read/write for a single CV bit on the main track (POM), building on the safe CV programming merged in #8.

New API on IZ21CommandStation

• WritePomCvBitAsync(loco, cv, bitPosition, bitValue, timeout) — writes one CV bit (LAN_X_CV_POM_WRITE_BIT, §6.7). Since a POM write returns no acknowledgement, it verifies by reading the CV byte back (LAN_X_CV_POM_READ_BYTE, §6.8, RailCom) and retrying until the target bit matches or the deadline elapses.
• ReadPomCvBitAsync(loco, cv, bitPosition, timeout) → bool — reads the CV byte back and returns the target bit.

Notes

• Reuses the hardened CV plumbing: the _cvLock serialization, Task.WaitAsync deadline bounding, OCE→timeout mapping, dispose handling, CvRetryDelay backoff, and short-circuit abort.
• bitPosition is validated to 0–7 (ArgumentOutOfRangeException otherwise).
• Requires RailCom for the read-back (same constraint as the POM byte write).

Tests

8 new unit tests (write-bit match / retry / timeout / invalid position; read-bit set / clear / timeout / invalid position). Full solution green: 710 passed, hardware smoke tests skipped.