Add signal capture and replay feature

Records raw demodulated CC1101 GDO0 output at 100 kHz into a 50 KB
bit-packed static buffer (up to 4 seconds). Replay drives GDO0 in
direct TX mode at the same sample rate, looping until stopped.

- config.h: CAP_SAMPLE_HZ / CAP_DURATION_S / CAP_BUF_BYTES defines
- main.cpp: capRecordISR / capReplayISR using hw_timer_t on timer 3
- main.cpp: startCapture / startReplay / stopCapture management functions
- main.cpp: capAnalyze() estimates bitrate and duty cycle from transitions
- main.cpp: five HTTP endpoints under /api/capture/*
- main.cpp: loop() state machine auto-finalises buffer-full capture
- kHtml: Capture / Replay card with freq input, radio selector,
  REC/STOP/REPLAY buttons, progress bar, stats grid, waveform canvas

Made-with: Cursor

include/config.h

@@ -84,4 +84,11 @@
 #define ENC_CLK_PIN   14
 #define ENC_DT_PIN    21
 
+// Signal capture / replay
+// Samples GDO0 (CC1101 demodulated output) at CAP_SAMPLE_HZ during direct RX mode.
+// Bit-packed into a static buffer. Replay drives GDO0 in direct TX mode at same rate.
+#define CAP_SAMPLE_HZ    100000                         // 100 kHz sample clock
+#define CAP_DURATION_S   4                              // max capture window (seconds)
+#define CAP_BUF_BYTES    ((CAP_SAMPLE_HZ * CAP_DURATION_S) / 8 + 8)  // ~50 KB
+
 #endif