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/document-features/Cargo.toml

@@ -0,0 +1,51 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2018"
+rust-version = "1.56"
+name = "document-features"
+version = "0.2.12"
+authors = ["Slint Developers <info@slint.dev>"]
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Extract documentation for the feature flags from comments in Cargo.toml"
+homepage = "https://slint.rs"
+readme = "README.md"
+keywords = [
+    "documentation",
+    "features",
+    "rustdoc",
+    "macro",
+]
+categories = ["development-tools"]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/slint-ui/document-features"
+
+[features]
+default = []
+self-test = []
+
+[lib]
+name = "document_features"
+path = "lib.rs"
+proc-macro = true
+
+[[test]]
+name = "self-doc"
+path = "tests/self-doc.rs"
+
+[dependencies.litrs]
+version = "1.0.0"