Runtime HSDAOH/MISRC backend selection, dual-ADC Channel B support and first-connect fix

[machcnz]

Summary

Single binary supports both HSDAOH and MISRC hardware modes, selectable at runtime
via the existing MISRC/HSDAOH toggle button. No recompile needed to switch backends.

Changes

Runtime backend selection

• MISRC/HSDAOH button now selects which capture backend is used at connect time
• Existing raw-parser and upstream callback paths unchanged, just wired to the toggle

Dual-ADC Channel B support (HSDAOH upstream path)

• Added stream_id=1 handler for dual AD9226 hardware (auto-detected on first callback)
  -- When dual-ADC detected, merges stream_id=0 (Ch A) and stream_id=1 (Ch B) into the same packed uint32 sample format that MISRC extraction already uses
• Extraction thread re-reads Channel B availability each loop iteration, so dual-ADC auto-detection during capture works without restarting extraction (simplest code change)
• Single-ADC hardware unaffected

Dual PCM1802 audio support

• Added stream_id=3 handler for second PCM1802 on dual-audio hsdaoh hardware

First-connect timeout fix

• Reset heartbeat baseline and added 5-second grace period after stream start
• Eliminates connect/disconnect/reconnect cycling on first USB connection

Other

• FFT grid labels now extend to 20 MHz (was capped at 11 MHz)
• Device connection and sync logging (device name, backend, single/dual-ADC hsdaoh)
• MinGW linker fix: crc16_simple static inline in vendored libhsdaoh

Testing

• hsdaoh: Single AD9226 + single PCM1802: verified working (Windows)
• hsdaoh: Dual AD9226 + dual PCM1802: implemented, untested (no hardware available yet)
• MISRC mode: unchanged, not affected