CAN decoder: add bit destuffing for real capture support

[vailuc]

Problem

The current decodeCAN implementation reads raw bit values without performing CAN bit destuffing. Per the CAN 2.0 spec, after 5 consecutive bits of the same value, a complement (stuff) bit is inserted. The decoder must remove these stuff bits to correctly parse the arbitration, control, data, and CRC fields.

Impact

• Real CAN captures (e.g. the can_mcp2515_125k_msg_222.sr fixture from sigrok-dumps) cannot be fully decoded — decodeCAN returns 0 frames.
• autoDetectLaProtocol falls back to UART instead of CAN on real captures.
• Label-based detection (inferDecoderDefaults with CAN_RX label) still works correctly.

Fix

Add a bit-destuffing pass in decodeCAN that:

1. Tracks consecutive same-valued bits
2. After 5 consecutive, skips the next stuff bit
3. Uses destuffed bit stream for all field parsing (ID, RTR, IDE, DLC, data, CRC)

Test

The existing can_mcp2515_125k_msg_222.sr fixture (4 MS/s, 125 kbits, standard frame ID 222 with 5 data bytes) can be used to verify the fix. The test in sigrokFixtures.test.ts already checks label detection and signal activity — once destuffing is added, it should be extended to verify full frame decode.