fix(dsp): a signal-free HARQ attempt voted at full strength (#1364)

CLAUDE.md

@@ -356,7 +356,7 @@ Gates: `hpx_hf_rungs_survive_fade` (fade), `goodput_gate` (the clean-channel cou
 
 **The rate ladder's SNR scales are per-waveform-family by physical necessity — do NOT try to unify them.** `rx_snr_db` dispatches to each plugin's `estimate_snr_db`, and they report different quantities: single-carrier PSK (BPSK, post-#934) reads ~true channel SNR; OFDM/SC-FDMA read a *saturation-bounded plugin-domain* SNR that flattens near ~16 dB (ZF noise-enhancement on faded subcarriers) and **physically cannot** report the 20–30 dB the dense rungs run at. So `hpx_hf`'s SL2–SL6 floors are true channel SNR and SL7–SL14 floors are plugin-domain — two scales, one ladder. This looks like a bug; it is not. Forcing OFDM onto a true-SNR scale (or "unifying the estimators") would put the top rungs' floors above anything the estimate can read → the SNR climb never reaches them → the exact v0.14.0 "AWGN-scale floors never clear" stall. The **evidence-based climb** (#934) is what makes two scales safe: it advances on decode success where SNR saturates. Gate: `snr_scale_boundary` — it fails if OFDM starts tracking true SNR without the floors being re-derived in the same change (the two are one decision). Full single-carrier→true-SNR unification of QPSK/8PSK was scoped and DECLINED: high-risk (floor recalibration across 5 profiles) for churn-reduction only, since the evidence climb already self-corrects the mismatch.
 
-**LLRs already carry `1/σ²` — do not weight them by it again.** `openpulse_dsp::constellation::symbol_llrs` divides every distance by `noise_var`, so a calibrated plugin (SC-FDMA, OFDM) emits true log-likelihood ratios whose magnitude is ∝ 1/σ². For repeated observations of the same bits, LLRs **add**: `combine_llrs_map` is the MAP combine and *is* inverse-noise weighting. The engine used to re-weight that sum by a `1 / mean(|LLR|)` proxy — a second 1/σ² — costing 0.75 dB on graded HARQ attempt sets (fixed in PR #686). `combine_llrs_weighted` is only for LLRs with a noise-blind scale (the ±1.0 trait default). Every shipped plugin is now calibrated (PR #687) **in the weak sense only, and the word hides two different bars**: `llr_calibration.rs` checks that `mean(|LLR|)` GROWS with SNR by a per-plugin floor (BPSK ≥ 8×, QPSK ≥ 1.1×, 8PSK ≥ 1.4×, ideal 15.8×) — that a σ² is divided by at all. The strong bar, that a bit carrying `|L|` is wrong about `1/(1+e^{|L|})` of the time within 4×, is checked only by the five `llr_reliability.rs` files (64QAM, MFSK16, OFDM, PILOT, SC-FDMA). **bpsk, qpsk and psk8 meet the weak bar and are untested against the strong one** — measured 2026-09-13, QPSK250 and 8PSK500 are in fact within ~1.25× at the SNRs they are used at, but nothing holds them there. Before citing "calibrated", say which bar. **Choosing the noise estimator is the hard part**: a demodulator's residual is not all thermal noise — pulse-shaping ISI and equalizer misadjustment vary the symbol *amplitude* with no SNR dependence, so a moment (M2/M4) or distance-to-nearest-point estimator stops tracking SNR entirely. Use the component *orthogonal* to the hard decision (`psk_symbol_noise_var`), or for a differential detector the quadrature companion, where the amplitude cancels exactly (`differential_llr_scale`).
+**LLRs already carry `1/σ²` — do not weight them by it again.** `openpulse_dsp::constellation::symbol_llrs` divides every distance by `noise_var`, so a calibrated plugin (SC-FDMA, OFDM) emits true log-likelihood ratios whose magnitude is ∝ 1/σ². For repeated observations of the same bits, LLRs **add**: `combine_llrs_map` is the MAP combine and *is* inverse-noise weighting. The engine used to re-weight that sum by a `1 / mean(|LLR|)` proxy — a second 1/σ² — costing 0.75 dB on graded HARQ attempt sets (fixed in PR #686). `combine_llrs_weighted` is only for LLRs with a noise-blind scale (the ±1.0 trait default). Every shipped plugin is now calibrated (PR #687) **in the weak sense only, and the word hides two different bars**: `llr_calibration.rs` checks that `mean(|LLR|)` GROWS with SNR by a per-plugin floor (BPSK ≥ 8×, QPSK ≥ 1.1×, 8PSK ≥ 1.4×, ideal 15.8×) — that a σ² is divided by at all. The strong bar, that a bit carrying `|L|` is wrong about `1/(1+e^{|L|})` of the time within 4×, is checked only by the five `llr_reliability.rs` files (64QAM, MFSK16, OFDM, PILOT, SC-FDMA). **bpsk, qpsk and psk8 meet the weak bar and are untested against the strong one** — measured 2026-09-13, QPSK250 and 8PSK500 are in fact within ~1.25× at the SNRs they are used at, but nothing holds them there. Before citing "calibrated", say which bar. **Choosing the noise estimator is the hard part**: a demodulator's residual is not all thermal noise — pulse-shaping ISI and equalizer misadjustment vary the symbol *amplitude* with no SNR dependence, so a moment (M2/M4) or distance-to-nearest-point estimator stops tracking SNR entirely. Use the component *orthogonal* to the hard decision (`psk_symbol_noise_var`), or for a differential detector the quadrature companion, where the amplitude cancels exactly (`differential_llr_scale`). **Corrected 2026-09-21 (#1364): `1/σ²` is the right target only where the symbol amplitude is KNOWN.** For a blind differential detector it is the high-SNR *limit* of the true slope `2A²/var(dot)`, and an estimator that holds it all the way down keeps voting at full confidence on an attempt carrying no signal — measured, a noise-only attempt emitted LLRs of std 1.41 at every σ from 0.1 to 2.0, and against the true pairwise DBPSK LLR that is 8× over-confident at −12 dB. That matters only where LLRs are SUMMED across attempts, which is `hpx_hf` SL2–SL5.
 
 **Acquire on the normalised correlation, not the unnormalised score.** `IqMatchedFilter::search`'s argmax favours high *energy*, on the reasoning that "a deep-fade low-energy window cannot win". When the **preamble** is the faded part that is exactly backwards — SC-FDMA lost frames to data-region windows 4896 samples later that merely shared the pilot comb (ρ = 0.994 at the true offset with energy 19.4, versus ρ = 0.657 with energy 83.0). Use `search_normalized` with an energy floor (ρ is meaningless on a silent window). This looked like "fade dynamics" for a release and was slated as a channel-estimate fix; **ablating** `smooth_ce` entirely (temporarily, to test the hypothesis) left the numbers bit-identical (PR #689) — the sync was the mechanism. `smooth_ce` itself stays in the SC-FDMA demod (an EMA channel-estimate smoother, live at 6 call sites in `plugins/scfdma/src/demodulate.rs`); it just wasn't the thing that was broken.
 

crates/openpulse-core/src/fec.rs

@@ -607,7 +607,9 @@ impl Default for SoftCombiner {
 ///
 /// # When this is the wrong function
 ///
-/// A true log-likelihood ratio `log P(b=0|y) / P(b=1|y)` already carries `1/σ²` — see
+/// A true log-likelihood ratio `log P(b=0|y) / P(b=1|y)` already carries its own noise scaling —
+/// `1/σ²` where the amplitude is known, and the vanishing slope `2A²/var(dot)` where it is blind
+/// and must be estimated (#1364). See
 /// `openpulse_dsp::constellation::symbol_llrs`, which divides every distance by `noise_var`. For
 /// independent observations of the same bit, the MAP combine of true LLRs is their plain **sum**:
 /// use [`combine_llrs_map`]. Passing `noise_var = σ²` here on top of already-calibrated LLRs applies