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/critical-section/docs/msrv.md

@@ -0,0 +1,30 @@
+# Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on all stable Rust versions going back to
+the version stated as MSRV in the README. It *might* compile with even older versions but
+that may change in any new patch release.
+
+## How the MSRV will be upgraded
+
+For `critical-section`, we do not consider upgrading the MSRV a strictly breaking change as defined by
+[SemVer](https://semver.org).
+
+We follow these rules when upgrading it:
+
+- We will not update the MSRV on any patch release: \_.\_.*Z*.
+- We may upgrade the MSRV on any *major* or *minor* release: *X*.*Y*.\_.
+- We may upgrade the MSRV in any preliminary version release (e.g. an `-alpha` release) as
+  these serve as preparation for the final release.
+- MSRV upgrades will be clearly stated in the changelog.
+
+This applies both to `0._._` releases as well as `>=1._._` releases.
+
+For example:
+
+For a given `x.y.z` release, we may upgrade the MSRV on `x` and `y` releases but not on `z` releases.
+
+If your MSRV upgrade policy differs from this, you are advised to specify the
+`critical-section` dependency in your `Cargo.toml` accordingly.
+
+See the [Rust Embedded Working Group MSRV RFC](https://github.com/rust-embedded/wg/blob/master/rfcs/0523-msrv-2020.md)
+for more background information and reasoning.