fix(snr): SNR-SANITY exclusion requires >=3-sample physical distribution — single-snapshot disagreement cannot blind the entry pick (rig F238/F239)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019btwFxDt7D19SZSqPYvZnk

Author: secup

docs/CHANGELOG.md

@@ -10,6 +10,21 @@ This log tracks all bug fixes and behavioral changes to prevent re-doing work du
 
 ---
 
+## 2026-07-13 — fix(snr): SNR-SANITY attribution threshold — the gate may only exclude with a ≥3-sample physical distribution (rig F238/F239: single-snapshot exclusion left entries BLIND at 15.0/none)
+
+At the handshake the physical ring holds 1-2 samples in an unknown fade state.
+F238/F239 (MPG@20): the connect frame's reading tripped the gate against that
+single snapshot, was excluded from rate selection, and the entry was picked
+with NO measurement at all — the protocol default 15.0 dB / source NONE →
+blind conservative DQPSK fallback (2 of 3 campaign entries). A snapshot cannot
+convict a meter: the exclusion now requires the same ≥3-sample trust bar the
+distribution display uses; below it the gate is silent. The F236 busy-channel
+detection (ring well-populated) is unaffected.
+
+Verified: ctest 85/85; good@20 PASS 1600 bps.
+
+---
+
 ## 2026-07-13 — fix(snr): LTS glitch guard — a synced frame at sub-0 dB is a measurement fault, not channel state (rig F237: meter collapsed to −9.2 at a 20 dB channel)
 
 The 07-10 estimation-noise debias floored at 0.01× when a glitched LTS pair

src/gui/modem/streaming_sync_acquisition.cpp

@@ -303,7 +303,19 @@ void StreamingDecoder::populateDecodeMetrics(DecodeResult& result, bool is_ofdm,
         // −0.66 dB noise-ref residual + both estimators' per-frame variance;
         // tighten alongside the §2 residual fix. This exact invariant caught
         // the pre-recalibration OFDM optimism from a single operator log line.
+        // ATTRIBUTION THRESHOLD (rig F238, 2026-07-13): the gate may only
+        // EXCLUDE a reading when its physical reference is a DISTRIBUTION
+        // (>=3 ring samples — the display's own trust bar), not a single
+        // snapshot. At the handshake the ring holds 1-2 samples in an unknown
+        // fade state; excluding the connect frame's ONLY reading left the
+        // entry pick blind (protocol default 15.0/source-none -> DQPSK
+        // fallback). Single-sample disagreement is logged by the WARN below
+        // when it trips with n>=3; below that, silence — a snapshot cannot
+        // convict a meter.
+        float ph_mean = 0.0f, ph_spread = 0.0f;
+        const size_t ph_n = physicalSnrStats(ph_mean, ph_spread);
         if (result.snr_source == SNRSource::MCDPSK_IN_BAND &&
+            ph_n >= 3 &&
             last_physical_snr_valid_.load() &&
             result.snr_db > last_physical_snr_db_.load() + 2.0f) {
             LOG_MODEM(WARN,