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.

Cargo.lock

@@ -1404,9 +1404,9 @@ checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
 [[package]]
 name = "hsmc"
-version = "0.1.0"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2d8cb8820e7ff5fa4fa4b5dbb78b9f59d26ccc8e3bcf69d8f4c4ce29ee01b93a"
+checksum = "960effec6ded897c013d5cde8fb7f09bf0137e783efc4c2e0346092a2941c46e"
 dependencies = [
  "embassy-futures",
  "embassy-sync 0.8.0",
@@ -1417,9 +1417,9 @@ dependencies = [
 
 [[package]]
 name = "hsmc-macros"
-version = "0.1.0"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c4fa4dad2c05309fae4f6d15b74d9b37c1fd7325a96701972fdad89637635160"
+checksum = "3e1933eea1e95edb4fa03688f969f0a1ffabc4f16fc396b40ead6bedd5c4bdff"
 dependencies = [
  "proc-macro2",
  "quote",