feat(cw): add greedy CTC collapse for DeepCW output

把模型 log_probs [batch,time,42] 输出折叠成文本, 对齐参考实现
greedy_ctc_decode: 逐帧取 argmax, 去 blank, 再去连续重复标签。

blank 的作用: 两个相同字母之间的 blank 是保住真实双字母的关键
(例如 "5NN" 的两个 N), 否则会被折叠成一个。

验证:
./gradlew :core:domain:test --tests '*CwCtcDecoderTest*'
=> tests="7" skipped="0" failures="0" errors="0"
含断言: 连续重复折叠、blank 隔断保留双字母、呼号 "CQ BG7" 含空格与数字

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/cw/CwCtcDecoder.kt

@@ -0,0 +1,61 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.core.domain.cw
+
+/**
+ * Turns the model's `log_probs` output into text.
+ *
+ * Mirrors the reference `greedy_ctc_decode`: take the best class per frame,
+ * drop blanks, and collapse runs of the same label. A blank between two
+ * identical labels is what keeps a genuine double letter (for example the
+ * two N's in "5NN") from collapsing into one.
+ */
+object CwCtcDecoder {
+
+    /**
+     * @param logProbs `[batch, time, class]`; only batch 0 is read.
+     * @param chars class index to symbol, excluding the blank.
+     * @param blankIndex the CTC blank class (41 for this model).
+     */
+    fun greedy(
+        logProbs: Array<Array<FloatArray>>,
+        chars: List<String>,
+        blankIndex: Int
+    ): String {
+        if (logProbs.isEmpty()) return ""
+        val frames = logProbs[0]
+        val builder = StringBuilder()
+        var previous = -1
+
+        for (frame in frames) {
+            var best = 0
+            for (i in 1 until frame.size) {
+                if (frame[i] > frame[best]) best = i
+            }
+            if (best == blankIndex) {
+                previous = -1
+                continue
+            }
+            if (best != previous && best < chars.size) {
+                builder.append(chars[best])
+            }
+            previous = best
+        }
+        return builder.toString()
+    }
+}

core/domain/src/test/java/com/rtbishop/look4sat/core/domain/cw/CwCtcDecoderTest.kt

@@ -0,0 +1,90 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.core.domain.cw
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+/**
+ * Greedy CTC collapse, matching the reference implementation's
+ * `greedy_ctc_decode`: drop blanks, then drop runs of the same label.
+ *
+ * Alphabet from `model.onnx.json` — 41 symbols plus blank at index 41.
+ */
+class CwCtcDecoderTest {
+
+    private val chars = listOf(
+        ",", ".", "/", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "?",
+        "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N",
+        "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", " "
+    )
+    private val blank = 41
+
+    /** Build a `[1, T, 42]` log-prob tensor whose argmax follows [path]. */
+    private fun logits(path: IntArray): Array<Array<FloatArray>> {
+        val frames = Array(path.size) { t ->
+            FloatArray(42) { -10f }.also { it[path[t]] = 0f }
+        }
+        return arrayOf(frames)
+    }
+
+    @Test
+    fun alphabetSizeMatchesModelMetadata() {
+        assertEquals("41 symbols + blank = 42 classes", 41, chars.size)
+    }
+
+    @Test
+    fun greedy_dropsRunsOfTheSameLabel() {
+        assertEquals("A", CwCtcDecoder.greedy(logits(intArrayOf(14, 14, 14)), chars, blank))
+    }
+
+    @Test
+    fun greedy_keepsRepeatsSeparatedByBlank() {
+        // A A <blank> A collapses to "AA": the blank breaks the run.
+        assertEquals("AA", CwCtcDecoder.greedy(logits(intArrayOf(14, 14, blank, 14)), chars, blank))
+    }
+
+    @Test
+    fun greedy_allBlanksYieldEmptyString() {
+        assertEquals("", CwCtcDecoder.greedy(logits(intArrayOf(blank, blank, blank)), chars, blank))
+    }
+
+    @Test
+    fun greedy_emptyInputYieldsEmptyString() {
+        assertEquals("", CwCtcDecoder.greedy(logits(intArrayOf()), chars, blank))
+    }
+
+    @Test
+    fun greedy_decodesCallsignWithSpaceAndDigits() {
+        // "CQ BG7" — C=16 Q=30 space=40 B=15 G=20 7=10
+        val path = intArrayOf(
+            blank, 16, 16, blank, 30, blank, 40,
+            15, blank, 20, blank, 10, blank
+        )
+        assertEquals("CQ BG7", CwCtcDecoder.greedy(logits(path), chars, blank))
+    }
+
+    @Test
+    fun greedy_picksHighestScoringClassPerFrame() {
+        // Frame favours S (32) over T (33); only S must survive.
+        val frame = FloatArray(42) { -10f }
+        frame[33] = -1f
+        frame[32] = -0.1f
+        assertEquals("S", CwCtcDecoder.greedy(arrayOf(arrayOf(frame)), chars, blank))
+    }
+}