Fix missing high nibble in POM CV bit-write data byte

[jaak0b]

Fixes a frame-encoding bug in the POM bit-write commands: DB5 was built as (V?0x08:0) | PPP, omitting the high nibble.

DB5 is the DCC bit-manipulation data byte, which NMRA S-9.2.1 defines as 111K-DBBB. For a write K=1, so the high nibble must be 1111 (0xF0). Without it the decoder receives an invalid opcode and silently ignores the frame — so a raw bit write does nothing on hardware.

Fixed (same defect in both)

• CvPomWriteBitCommand (§6.7, locomotive) — also corrects the frame emitted by the safe WritePomCvBitAsync wrapper.
• CvPomAccessoryWriteBitCommand (§6.10, accessory) — identical bug.

DB5 is now 0xF0 | (V?0x08:0) | (PPP & 0x07).

Tests

Updated both byte-exact test fixtures (DB5 and recomputed XOR). The byte read/write and bit read paths were already correct and are unaffected. Full solution green: 710 passed.