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/byteorder/Cargo.toml

@@ -0,0 +1,54 @@
+# 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 = "2021"
+rust-version = "1.60"
+name = "byteorder"
+version = "1.5.0"
+authors = ["Andrew Gallant <jamslam@gmail.com>"]
+description = "Library for reading/writing numbers in big-endian and little-endian."
+homepage = "https://github.com/BurntSushi/byteorder"
+documentation = "https://docs.rs/byteorder"
+readme = "README.md"
+keywords = [
+    "byte",
+    "endian",
+    "big-endian",
+    "little-endian",
+    "binary",
+]
+categories = [
+    "encoding",
+    "parsing",
+    "no-std",
+]
+license = "Unlicense OR MIT"
+repository = "https://github.com/BurntSushi/byteorder"
+
+[profile.bench]
+opt-level = 3
+
+[lib]
+name = "byteorder"
+bench = false
+
+[dev-dependencies.quickcheck]
+version = "0.9.2"
+default-features = false
+
+[dev-dependencies.rand]
+version = "0.7"
+
+[features]
+default = ["std"]
+i128 = []
+std = []