feat(display): OLED dims a few seconds after last activity

Splash holds bright for 10 s on (re-)entry, then drops to dim while
LearnMore/Info keep alternating. The dashboard stays bright for 3 s
after the last packet or mode change, then dims in place without
disturbing the Listening/Transmitting state or the 1 s sparkline
timer. Cross-leaf TX↔RX hops stay bright throughout — `set_dim` is
fired by the 3 s timer handler as an action rather than as a sibling
`exit:` action, so mode-change leaf-crossings no longer flash
dim→bright on every swap.

Two driver families (`ssd1306::Ssd1306Async` and the in-tree
`Sh1106`) wrapped behind a single `DisplayBrightness` trait in
`src/driver/mod.rs`; the board trait's `DisplayDriver` associated
type now requires it. Dim/bright contrast values centralized as
`CONTRAST_BRIGHT` / `CONTRAST_DIM` constants — tune in one place.

`On.on(CmdReset)` now targets `LearnMore` directly rather than the
ancestor `Splash`. Under hsmc 0.2 up-transition semantics, targeting
an already-active ancestor unwinds only below it without re-descent;
the USB-disconnect reset path was previously relying on the old
re-enter-and-descend behavior and would have parked at `Splash`
with a dead alternation timer.

Bumps `hsmc` 0.1.0 → 0.2 for the up-transition semantic fix.

src/driver/mod.rs

@@ -10,3 +10,56 @@ pub mod sh1106;
     feature = "waveshare_rp2040_lora"
 ))]
 pub mod simple_led;
+
+// ── Display brightness ────────────────────────────────────────────────
+//
+// The display statechart dims the OLED a few seconds after the last
+// activity. Our two driver families (`ssd1306` crate and our in-tree
+// `sh1106`) expose incompatible brightness APIs, so we wrap them in a
+// single async trait that the state-machine actions can call uniformly.
+
+/// Two-level backlight control used by the display dashboard.
+pub trait DisplayBrightness {
+    fn set_bright(&mut self) -> impl core::future::Future<Output = ()>;
+    fn set_dim(&mut self) -> impl core::future::Future<Output = ()>;
+}
+
+// Contrast values for the two brightness levels, picked to be common across
+// both driver families. `0xFF` is the SSD1306/SH1106 maximum; `0x08` is ~3%
+// of max — visible in a dim room, barely noticeable in daylight. Tune here.
+const CONTRAST_BRIGHT: u8 = 0xFF;
+const CONTRAST_DIM: u8 = 0x08;
+
+impl<DI, SIZE, MODE> DisplayBrightness for ssd1306::Ssd1306Async<DI, SIZE, MODE>
+where
+    DI: display_interface::AsyncWriteOnlyDataCommand,
+    SIZE: ssd1306::size::DisplaySizeAsync,
+{
+    async fn set_bright(&mut self) {
+        // `Brightness::custom(precharge, contrast)`. Matches the shape of
+        // the crate's BRIGHTEST preset (precharge 0x2) at max contrast.
+        let _ = self
+            .set_brightness(ssd1306::prelude::Brightness::custom(0x2, CONTRAST_BRIGHT))
+            .await;
+    }
+    async fn set_dim(&mut self) {
+        // Low precharge pairs with low contrast — same combination the
+        // crate's DIMMEST preset uses.
+        let _ = self
+            .set_brightness(ssd1306::prelude::Brightness::custom(0x1, CONTRAST_DIM))
+            .await;
+    }
+}
+
+#[cfg(any(feature = "wio_tracker_l1", feature = "elecrow_thinknode_m2"))]
+impl<I> DisplayBrightness for sh1106::Sh1106<I>
+where
+    I: embedded_hal_async::i2c::I2c,
+{
+    async fn set_bright(&mut self) {
+        let _ = self.set_brightness(CONTRAST_BRIGHT).await;
+    }
+    async fn set_dim(&mut self) {
+        let _ = self.set_brightness(CONTRAST_DIM).await;
+    }
+}