refactor: drive display via hsmc statechart

Replaces the ad-hoc display loop with a declarative hsmc statechart
(Off/On.{Splash.{LearnMore,Info}, Dashboard.{Listening,Transmitting}})
and swaps the Radio→Display channel from a RadioStatus watch to a
typed RadioEvent queue. The radio task now emits discrete mode and
packet events instead of publishing full status snapshots; the
display consumes them through the statechart, which also handles the
5 s splash alternation and 1 s sparkline tick via `on(after …)` /
`on(every …)` transitions.

Also:
- Add hsmc dep and embassy/tokio feature shims (hsmc proc macro
  evaluates `cfg(feature = "embassy")` in this crate).
- Fix USB disconnect path in `host/usb.rs` so the DTR branch always
  runs on read error (was early-continuing, starving display/radio
  reset).

Cargo.lock

@@ -514,6 +514,7 @@ dependencies = [
  "esp-println",
  "esp-rtos",
  "heapless 0.9.2",
+ "hsmc",
  "lora-phy",
  "minijinja",
  "owo-colors",
@@ -1398,6 +1399,26 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
+[[package]]
+name = "hsmc"
+version = "0.1.0"
+dependencies = [
+ "embassy-futures",
+ "embassy-sync 0.8.0",
+ "embassy-time",
+ "heapless 0.8.0",
+ "hsmc-macros",
+]
+
+[[package]]
+name = "hsmc-macros"
+version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "ident_case"
 version = "1.0.1"