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

feature/cw/src/main/java/com/rtbishop/look4sat/feature/cw/CwDecodeScreen.kt

@@ -94,6 +94,7 @@ fun CwDecodeScreen(navigateUp: () -> Unit = {}) {
     var isListening by remember { mutableStateOf(false) }
 
     val decodedText by decoder.decodedText.collectAsState()
+    val historyText by decoder.historyText.collectAsState()
     val estimatedPitch by decoder.estimatedPitch.collectAsState()
     val signalStrength by decoder.signalStrength.collectAsState()
     val inferenceMs by decoder.lastInferenceMs.collectAsState()
@@ -207,7 +208,7 @@ fun CwDecodeScreen(navigateUp: () -> Unit = {}) {
                 .background(MaterialTheme.colorScheme.surfaceVariant.copy(alpha = 0.4f))
         ) {
             Text(
-                text = decodedText,
+                text = (historyText + decodedText).ifEmpty { "…" },
                 modifier = Modifier
                     .fillMaxSize()
                     .verticalScroll(rememberScrollState())

feature/cw/src/main/java/com/rtbishop/look4sat/feature/cw/CwWaterfall.kt

@@ -25,6 +25,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import com.rtbishop.look4sat.core.domain.cw.CwDeepSpectrogram
 import kotlinx.coroutines.flow.MutableStateFlow
@@ -96,7 +97,7 @@ class CwWaterfallState(private val historyRows: Int = 96) {
 
 /**
  * Draws the waterfall newest-row-last, one pixel column per frequency bin.
- * Colour ramp goes dark blue -> cyan -> yellow with magnitude.
+ * Colour ramp is the inferno palette (black -> purple -> orange -> yellow).
  */
 @Composable
 internal fun CwWaterfallView(
@@ -126,11 +127,14 @@ internal fun CwWaterfallView(
 
         for ((index, row) in rows.withIndex()) {
             val y = index * rowHeight
-            for (bin in row.indices) {
-                val magnitude = (row[bin] / peak).coerceIn(0f, 1f)
-                if (magnitude < 0.06f) continue
+            // Linear interpolation between adjacent bins via a horizontal
+            // gradient removes the blocky "pixel" look of 65 discrete columns.
+            for (bin in 0 until row.size - 1) {
+                val m0 = (row[bin] / peak).coerceIn(0f, 1f)
+                val m1 = (row[bin + 1] / peak).coerceIn(0f, 1f)
+                if (m0 < 0.06f && m1 < 0.06f) continue
                 drawRect(
-                    color = rampColor(magnitude),
+                    brush = Brush.horizontalGradient(listOf(inferno(m0), inferno(m1))),
                     topLeft = Offset(bin * binWidth, y),
                     size = Size(binWidth + 1f, rowHeight + 1f)
                 )
@@ -147,13 +151,32 @@ internal fun CwWaterfallView(
     }
 }
 
-private fun rampColor(magnitude: Float): Color = when {
-    magnitude < 0.5f -> {
-        val t = magnitude / 0.5f
-        Color(red = 0f, green = 0.35f * t, blue = 0.35f + 0.55f * t)
-    }
-    else -> {
-        val t = (magnitude - 0.5f) / 0.5f
-        Color(red = t, green = 0.35f + 0.6f * t, blue = 0.9f - 0.8f * t)
+/**
+ * matplotlib "inferno" colour map, approximated with piecewise-linear stops
+ * (black -> purple -> magenta-red -> orange -> pale yellow). The same palette
+ * used for the static spectrogram illustration, kept for visual consistency.
+ */
+private val INFERNO_STOPS = arrayOf(
+    floatArrayOf(0.00f, 0.000f, 0.000f, 0.016f), // black
+    floatArrayOf(0.25f, 0.231f, 0.059f, 0.439f), // deep purple
+    floatArrayOf(0.50f, 0.549f, 0.161f, 0.506f), // magenta
+    floatArrayOf(0.75f, 0.871f, 0.286f, 0.408f), // red-orange
+    floatArrayOf(1.00f, 0.988f, 1.000f, 0.643f)  // pale yellow
+)
+
+private fun inferno(t: Float): Color {
+    val x = t.coerceIn(0f, 1f)
+    for (i in 0 until INFERNO_STOPS.size - 1) {
+        val a = INFERNO_STOPS[i]
+        val b = INFERNO_STOPS[i + 1]
+        if (x <= b[0]) {
+            val f = (x - a[0]) / (b[0] - a[0])
+            return Color(
+                red = a[1] + (b[1] - a[1]) * f,
+                green = a[2] + (b[2] - a[2]) * f,
+                blue = a[3] + (b[3] - a[3]) * f
+            )
+        }
     }
+    return Color(0.988f, 1.0f, 0.643f)
 }