[fix](trx-rds): tune RDS parameters for maximum sensitivity

- RRC_ALPHA 0.75→0.50: narrower noise BW, ~0.6 dB SNR gain
- COSTAS_KI 3.5e-7: maintain ζ≈0.68 (1e-6 caused loop instability)
- Soft confidence: use biphase_i.abs() instead of full vector magnitude
  so OSD confidence is aligned with bit-decision sign; suppresses
  false groups under noise with residual Costas phase error
- OSD(2) in locked mode: corrects ≤2-bit errors after block sync
- Search mode: hard decode only for Block A; OSD(1) in search yielded
  ~13% false Block A rate per bit, letting wrong clock candidates
  accumulate false groups as fast as the correct candidate
- Incumbent candidate tracking (best_candidate_idx): the winning
  candidate updates best_state at equal score; challengers need strictly
  higher score; best_score tracks incumbent even on no-state-change
  groups so challengers can't leapfrog on a single false group
- blocks_to_chips: add NRZI (NRZ-Mark) pre-encoding so the differential
  biphase decoder recovers actual data bits rather than XOR-of-pairs
- Add blocks_to_chips_round_trips_all_groups test: verifies all 16 blocks
  across 4 PS segments round-trip correctly without BPSK modulation

[fix](trx-backend-soapysdr): lower pilot lock threshold for weak-signal RDS

- PILOT_LOCK_THRESHOLD 0.25→0.20, add PILOT_LOCK_ONSET=0.30 constant
- Pilot reference engages at coherence ≥0.36 (was ≥0.45)

WIP: end_to_end_clean_signal_decodes_ps still failing (13/15 pass).
Decoder skips segment 2 due to ISI from rectangular test chips through
RRC receive filter. chips_to_rds_signal needs RRC pulse shaping.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -2760,6 +2760,7 @@ dependencies = [
 name = "trx-rds"
 version = "0.1.0"
 dependencies = [
+ "rustfft",
  "trx-core",
 ]