Fix false-sync escalation blocking ACK decode and two OFDM code bugs

- Add LLR quality gate before 4-CW escalation in streaming decoder:
  false syncs from fading-corrupted LTS produce |llr|_avg ~1.0, while
  legitimate interleaved data has |llr|_avg > 3.0. Escalating on garbage
  wasted ~1.5s on 4x failed LDPC (50 iters each), blocking the decoder
  from processing real ACK frames and causing cascading timeout retransmissions.
  R1/4 file transfer retransmissions: 21 -> 8, timeouts: 18 -> 0.

- Fix ConnectFrame re-encode using wrong code rate (streaming_encoder.cpp):
  after patching total_cw field, re-encode incorrectly used code_rate_
  instead of CodeRate::R1_4 for control frames.

- Deduplicate PILOT_RNG_SEED constant (modulator.cpp):
  removed duplicate definition, now uses single source from
  demodulator_constants.hpp. Divergence would silently break TX/RX
  pilot sequence matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: secup

src/gui/modem/streaming_decoder.cpp

@@ -1174,10 +1174,35 @@ void StreamingDecoder::decodeCurrentFrame() {
         }
 
         if (!peek_fell_through) {
+            // Check LLR quality before expensive 4-CW escalation.
+            // False syncs (e.g. from fading-corrupted LTS) produce low |llr|_avg (~1.0),
+            // while legitimate interleaved data frames have |llr|_avg > 3.0.
+            // Escalating on garbage wastes ~1.5s on 4x failed LDPC (50 iters each),
+            // blocking the decoder from processing real frames arriving in the buffer.
+            float llr_abs_sum = 0.0f;
+            size_t llr_count = std::min(soft_bits.size(), LDPC_BLOCK);
+            for (size_t i = 0; i < llr_count; ++i) {
+                llr_abs_sum += std::abs(soft_bits[i]);
+            }
+            float llr_abs_avg = llr_abs_sum / static_cast<float>(llr_count);
+
+            constexpr float MIN_LLR_FOR_ESCALATION = 3.0f;
+            if (llr_abs_avg < MIN_LLR_FOR_ESCALATION) {
+                LOG_MODEM(INFO, "[%s] OFDM CW0 peek: decode failed, |llr|_avg=%.1f too low — skipping 4-CW escalation (likely false sync)",
+                          log_prefix_.c_str(), llr_abs_avg);
+                {
+                    std::lock_guard<std::mutex> lock(buffer_mutex_);
+                    correlation_pos_ = (sync_position_ + frame_len) % MAX_BUFFER_SAMPLES;
+                    last_decoded_sync_pos_ = sync_position_;
+                }
+                state_ = DecoderState::SEARCHING;
+                return;
+            }
+
             pending_total_cw_ = v2::FIXED_FRAME_CODEWORDS;  // 4
             state_ = DecoderState::SYNC_FOUND;
-            LOG_MODEM(INFO, "[%s] OFDM CW0 peek: decode failed, escalating to %d CWs",
-                      log_prefix_.c_str(), pending_total_cw_);
+            LOG_MODEM(INFO, "[%s] OFDM CW0 peek: decode failed (|llr|_avg=%.1f), escalating to %d CWs",
+                      log_prefix_.c_str(), llr_abs_avg, pending_total_cw_);
             return;
         }
     }

src/gui/modem/streaming_encoder.cpp

@@ -609,8 +609,8 @@ Bytes StreamingEncoder::encodeFrameBytes(const Bytes& frame_data) {
             uint16_t fcrc = v2::ControlFrame::calculateCRC(tx_data.data(), fcrc_offset);
             tx_data[fcrc_offset] = (fcrc >> 8) & 0xFF;
             tx_data[fcrc_offset + 1] = fcrc & 0xFF;
-            // Re-encode with patched header
-            cws = v2::encodeFrameWithLDPC(tx_data, code_rate_);
+            // Re-encode with patched header (always R1/4 for control frames)
+            cws = v2::encodeFrameWithLDPC(tx_data, CodeRate::R1_4);
         }
 
         Bytes encoded;

src/ofdm/modulator.cpp

@@ -3,6 +3,7 @@
 #include "ultra/ofdm.hpp"
 #include "ultra/dsp.hpp"
 #include "ultra/logging.hpp"
+#include "demodulator_constants.hpp"
 #include <random>
 
 namespace ultra {
@@ -34,10 +35,6 @@ Complex qam16_point(int bits) {
     return Complex(levels[i_bits] * QAM16_SCALE, levels[q_bits] * QAM16_SCALE);
 }
 
-// Pilot sequence RNG seed - must match between modulator and demodulator
-// Using a fixed seed ensures TX/RX pilot sequences are identical
-constexpr uint32_t PILOT_RNG_SEED = 0x50494C54;  // "PILT" in ASCII
-
 // 32-QAM rectangular constellation (5 bits per symbol)
 // Uses 8×4 grid: 8 Q levels × 4 I levels = 32 points
 // Gray-coded for minimum bit errors on adjacent symbol errors
@@ -194,7 +191,7 @@ struct OFDMModulator::Impl {
 
         // Pilot sequence: known BPSK pattern (pseudo-random but deterministic)
         pilot_sequence.resize(pilot_carrier_indices.size());
-        std::mt19937 rng(PILOT_RNG_SEED);
+        std::mt19937 rng(demod_constants::PILOT_RNG_SEED);
         for (size_t i = 0; i < pilot_sequence.size(); ++i) {
             pilot_sequence[i] = (rng() & 1) ? Complex(1, 0) : Complex(-1, 0);
         }