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/heck/CHANGELOG.md

@@ -0,0 +1,25 @@
+# unreleased
+
+# 0.5.0
+
+- Add `no_std` support.
+- Remove non-additive `unicode` feature. The library now uses `char::is_alphanumeric`
+  instead of the `unicode-segmentation` library to determine word boundaries in all cases.
+
+# 0.4.1
+
+Improvements:
+
+- Add Train-Case support
+
+# 0.4.0
+
+Breaking changes:
+
+* Make unicode support optional (off by default). Enable the `unicode` crate
+  feature if you need unicode support.
+* Rename all traits from `SomeCase` to `ToSomeCase`, matching `std`s convention
+  of beginning trait names with a verb (`ToOwned`, `AsRef`, …)
+* Rename `ToMixedCase` to `ToLowerCamelCase`
+* Rename `ToCamelCase` to `ToUpperCamelCase`
+* Add `ToPascalCase` as an alias to `ToUpperCamelCase`