feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/az/RELEASES.md

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+<!-- Copyright © 2019–2021 Trevor Spiteri -->
+
+<!-- Copying and distribution of this file, with or without
+modification, are permitted in any medium without royalty provided the
+copyright notice and this notice are preserved. This file is offered
+as-is, without any warranty. -->
+
+Version 1.2.1 (2022-07-25)
+==========================
+
+  * Fix build issue using rustc 1.64.0-nightly under Windows.
+
+Version 1.2.0 (2021-11-24)
+==========================
+
+  * The following traits were added, which can be used for constraints in the
+    opposite direction to the other cast traits.
+      * [CastFrom][cf-1-2]
+      * [CheckedCastFrom][ccf-1-2]
+      * [SaturatingCastFrom][scf-1-2]
+      * [WrappingCastFrom][wcf-1-2]
+      * [OverflowingCastFrom][ocf-1-2]
+      * [UnwrappedCastFrom][ucf-1-2]
+
+[ccf-1-2]: https://docs.rs/az/~1.2/az/trait.CheckedCastFrom.html
+[cf-1-2]: https://docs.rs/az/~1.2/az/trait.CastFrom.html
+[ocf-1-2]: https://docs.rs/az/~1.2/az/trait.OverflowingCastFrom.html
+[scf-1-2]: https://docs.rs/az/~1.2/az/trait.SaturatingCastFrom.html
+[ucf-1-2]: https://docs.rs/az/~1.2/az/trait.UnwrappedCastFrom.html
+[wcf-1-2]: https://docs.rs/az/~1.2/az/trait.WrappingCastFrom.html
+
+Version 1.1.2 (2021-08-23)
+==========================
+
+  * Now the [`Debug`] implementation for [`Round`][r-1-1] outputs the value only
+    without “`Round()`” around it.
+
+Version 1.1.1 (2021-03-25)
+==========================
+
+  * The `track_caller` attribute is now applied to panicking functions
+    if supported by the compiler.
+
+Version 1.1.0 (2021-02-03)
+==========================
+
+  * Unwrapped casts were added, which panic on overflow even when
+    debug assertions are not enabled.
+      * [UnwrappedCast][uc-1-1]
+      * [UnwrappedAs][ua-1-1]
+      * [unwrapped_cast][ucf-1-1]
+
+[r-1-1]: https://docs.rs/az/~1.1/az/struct.Round.html
+[ua-1-1]: https://docs.rs/az/~1.1/az/trait.UnwrappedAs.html
+[uc-1-1]: https://docs.rs/az/~1.1/az/trait.UnwrappedCast.html
+[ucf-1-1]: https://docs.rs/az/~1.1/az/fn.unwrapped_cast.html
+
+Version 1.0.0 (2020-04-18)
+==========================
+
+  * All deprecated items were removed.
+
+Version 0.3.1 (2020-04-17)
+==========================
+
+  * Static casts were deprecated as their use case was unclear.
+
+Version 0.3.0 (2019-10-01)
+==========================
+
+  * The behavior of static casts was changed: now they return
+    `Option`, but an implementation should either always return `Some`
+    or always return `None`.
+  * Bug fix: checked casts from floating-point to wrapped integers
+    were panicking for infinite or NaN.
+
+Version 0.2.0 (2019-09-10)
+==========================
+
+  * The old `*As` traits were renamed to `*Cast`.
+  * New more convenient `*As` traits were added.
+
+Version 0.1.0 (2019-09-09)
+==========================
+
+  * Conversions between integers and floating-point numbers.
+  * Checked, saturating, wrapping and overflowing conversions.
+  * Static conversions when the conversion cannot fail.
+  * Rounding conversions from floating-point numbers to integers using `Round`.
+
+[`Debug`]: https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html