feat(cw): archive decoded history, polish the waterfall, document licensing

三个用户反馈一并解决:

1. 解码文字不再消失 (核心)
   旧实现: 20 秒环形缓冲满了就静默覆盖最旧样本, 文字随之从屏幕消失。
   新实现: CwDeepBuffer 新增 overflow —— 满时被覆盖的旧样本先进 overflow,
   解码器累积到 15 秒就单独解码一次, 结果追加到 historyText (只增不减)。
   UI 记录区显示 historyText + decodedText (历史稳定 + 当前窗口实时)。
   ICwDecoder 接口新增 historyText StateFlow。

   归档音频已离开主窗口, 不再被 CTC 修正, 故其文本是"最终版", 追加安全。
   归档窗口 15 秒: 内容已在 20 秒窗口里解过多次, 短一点几乎无损, 且推理
   开销小。

2. 瀑布图更好看
   配色从"深蓝->青->黄"换成 matplotlib inferno (黑->紫->品红->橙->黄),
   与静态频谱图保持一致。相邻 bin 之间用水平渐变做线性插值, 消除 65 列
   离散方块的像素感。

3. AGPL 合规补漏 (用户提醒: 仓库许可证没体现 AGPL 组件)
   README 新增 License 章节: 声明项目主体 GPL-3.0 + feature/cw 的 DeepCW
   模型 AGPL-3.0-only, 并说明合并作品按 GPL-3.0 §13 / AGPL-3.0 §13 处理。

验证:
- :core:domain:test => 31 个 CW 测试全绿 (CwDeepBufferTest 新增 3 个
  overflow 归档测试: 顺序/清空/reset)
- :core:domain:compileKotlin + :core:data + :feature:cw:compileDebugKotlin
  => BUILD SUCCESSFUL

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

@@ -54,9 +54,22 @@ class CwDeepBuffer(
     private var filled = 0
     private var sinceLastRedecode = 0
 
+    /**
+     * Samples evicted from the ring once it is full. They are the audio that
+     * has scrolled out of the 20 s window, and are handed off (via
+     * [drainOverflow]) so the decoder can archive them into permanent history
+     * instead of silently dropping the corresponding text. Pre-allocated to
+     * [capacity]: overflow never exceeds one window before it is drained.
+     */
+    private val overflow = FloatArray(capacity)
+    private var overflowSize = 0
+
     /** Samples currently buffered, never above [capacity]. */
     val size: Int get() = filled
 
+    /** Samples currently held in the overflow (awaiting archival). */
+    val overflowCount: Int get() = overflowSize
+
     /** True once there is enough audio for the spectrogram to yield a frame. */
     val hasEnoughAudio: Boolean get() = filled >= CwDeepSpectrogram.FFT_LENGTH
 
@@ -71,10 +84,15 @@ class CwDeepBuffer(
             // A chunk longer than the window can only contribute its tail.
             val start = maxOf(0, chunk.size - capacity)
             for (i in start until chunk.size) {
+                if (filled == capacity) {
+                    // The slot we are about to overwrite holds the oldest
+                    // sample — move it to the overflow for archival.
+                    overflow[overflowSize++] = ring[writeIndex]
+                }
                 ring[writeIndex] = chunk[i]
                 writeIndex = (writeIndex + 1) % capacity
+                if (filled < capacity) filled++
             }
-            filled = minOf(capacity, filled + (chunk.size - start))
         }
 
         sinceLastRedecode += chunk.size
@@ -98,11 +116,25 @@ class CwDeepBuffer(
         return out
     }
 
-    /** Drop all audio and restart the re-decode interval. */
+    /**
+     * Return the evicted samples (chronological order) and clear the overflow.
+     * Safe to call every append; returns an empty array when nothing has been
+     * evicted yet.
+     */
+    fun drainOverflow(): FloatArray {
+        if (overflowSize == 0) return FloatArray(0)
+        val out = overflow.copyOf(overflowSize)
+        overflowSize = 0
+        return out
+    }
+
+    /** Drop all audio (including pending overflow) and restart the interval. */
     fun reset() {
         writeIndex = 0
         filled = 0
         sinceLastRedecode = 0
+        overflowSize = 0
         ring.fill(0f)
+        overflow.fill(0f)
     }
 }

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

@@ -28,7 +28,7 @@ import kotlinx.coroutines.flow.StateFlow
 interface ICwDecoder {
 
     /**
-     * Decoded text for display.
+     * Decoded text for the *current* window.
      *
      * Note this is **replace** semantics, not append: a whole-segment model
      * revises earlier characters as more audio arrives, so consumers must show
@@ -36,6 +36,13 @@ interface ICwDecoder {
      */
     val decodedText: StateFlow<String>
 
+    /**
+     * Permanent transcript of everything that has scrolled out of the live
+     * window. Unlike [decodedText] this only ever grows (until [reset]); it is
+     * what the user reads back after a signal has passed.
+     */
+    val historyText: StateFlow<String>
+
     /** Detected tone frequency in Hz, or null before a tone is found. */
     val estimatedPitch: StateFlow<Float?>
 

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

@@ -118,4 +118,35 @@ class CwDeepBufferTest {
         val buffer = CwDeepBuffer()
         assertEquals(CwDeepSpectrogram.SAMPLE_RATE * 20, buffer.capacity)
     }
+
+    @Test
+    fun overflowCollectsEvictedSamplesInOrder() {
+        val buffer = CwDeepBuffer(sampleRate = 4, maxSeconds = 1.0) // capacity 4
+        buffer.append(floatArrayOf(1f, 2f, 3f, 4f))
+        assertEquals("nothing evicted before the window is full", 0, buffer.overflowCount)
+        buffer.append(floatArrayOf(5f, 6f)) // overwrites 1, 2
+        assertArrayEquals("evicted samples, oldest first", floatArrayOf(1f, 2f), buffer.drainOverflow(), 0f)
+        assertArrayEquals("live window still correct", floatArrayOf(3f, 4f, 5f, 6f), buffer.snapshot(), 0f)
+    }
+
+    @Test
+    fun drainOverflowClearsItself() {
+        val buffer = CwDeepBuffer(sampleRate = 4, maxSeconds = 1.0)
+        buffer.append(floatArrayOf(1f, 2f, 3f, 4f))
+        buffer.append(floatArrayOf(5f))
+        assertEquals(1, buffer.overflowCount)
+        buffer.drainOverflow()
+        assertEquals(0, buffer.overflowCount)
+        assertArrayEquals(FloatArray(0), buffer.drainOverflow(), 0f)
+    }
+
+    @Test
+    fun resetClearsOverflow() {
+        val buffer = CwDeepBuffer(sampleRate = 4, maxSeconds = 1.0)
+        buffer.append(floatArrayOf(1f, 2f, 3f, 4f))
+        buffer.append(floatArrayOf(5f))
+        assertEquals(1, buffer.overflowCount)
+        buffer.reset()
+        assertEquals(0, buffer.overflowCount)
+    }
 }