Add physical and USB pairing management

bootsel_pairing_button.h

@@ -7,17 +7,26 @@ enum class BootselPairingButtonSample : uint8_t {
     kReleased,
     kPressed,
 };
+enum class BootselPairingButtonEvent : uint8_t {
+    kNone,
+    kOpenPairing,
+    kClearPairings,
+};
+
 
 class BootselPairingButtonHoldFsm {
 public:
-    static constexpr uint8_t kHoldSamples = 20;
+    static constexpr uint8_t kPairingHoldSamples = 20;
+    static constexpr uint8_t kClearHoldSamples = 100;
 
-    bool update(BootselPairingButtonSample sample);
+    BootselPairingButtonEvent update(BootselPairingButtonSample sample);
 
 private:
     uint8_t pressed_samples_ = 0;
-    bool hold_reported_ = false;
+    bool pairing_reported_ = false;
+    bool clear_reported_ = false;
 };
 
-// Polls BOOTSEL at 10 Hz. Returns true once when a 20-sample hold completes.
-bool bootsel_pairing_button_task();
+// Polls BOOTSEL at 10 Hz. Reports pairing at 2 seconds and clearing at
+// 10 seconds; each event fires once per continuous hold.
+BootselPairingButtonEvent bootsel_pairing_button_task();