fix(radio): guarantee TX_DONE/TX_FAILED reply on every CmdTx via Transmitting exit action

Add exit: respond_tx_failed on the Transmitting super-state. Per hsmc spec
§6, exit actions fire on every departure from a state, including the
Operational-level on(WedgeDetected) => Recovering transition that bypasses
Transmitting's own on(TransmitFailed) => respond_tx_failed handler.

respond_tx_failed is a clean no-op when last_tx_tag is None (the
normal-completion path consumed it via .take()), so this is a safe
unconditional exit action. Net effect: every CmdTx gets exactly one
tag-correlated TX_DONE/TX_FAILED/CHANNEL_BUSY reply no matter how
Transmitting is exited. The host (mux/ai-bot) stops eating 2s firmware-
timeouts on every CAD wedge mid-TX.

src/radio.rs

@@ -418,6 +418,19 @@ statechart! {
 
                 state Transmitting {
                     entry: announce_entered_tx;
+                    // Per hsmc README rule §6, exit actions fire on every
+                    // departure from Transmitting — including the
+                    // Operational-level `on(WedgeDetected) => Recovering`
+                    // transition that bypasses Transmitting's own
+                    // `on(TransmitFailed) => respond_tx_failed` handler.
+                    // Without this the host never gets a TX_DONE/TX_FAILED
+                    // reply on a wedge mid-TX and waits its 2 s firmware-
+                    // timeout. `respond_tx_failed` no-ops if `last_tx_tag`
+                    // is None (the normal-completion path consumed it via
+                    // `take()`), so it's a clean tag-correlated reply
+                    // GUARANTEE: every CmdTx gets a response no matter
+                    // how Transmitting is exited.
+                    exit: respond_tx_failed;
                     exit: announce_packet_tx;
                     default(TransmittingFromIdle);