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/micromath/Cargo.toml

@@ -0,0 +1,55 @@
+# 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"
+name = "micromath"
+version = "2.1.0"
+authors = ["Tony Arcieri <bascule@gmail.com>"]
+description = """
+Embedded-friendly math library featuring fast floating point approximations
+(with small code size) for common arithmetic operations, trigonometry,
+2D/3D vector types, statistical analysis, and quaternions.
+Optimizes for performance and small code size at the cost of precision.
+"""
+readme = "README.md"
+keywords = [
+    "math",
+    "quaternions",
+    "statistics",
+    "trigonometry",
+    "vector",
+]
+categories = [
+    "embedded",
+    "mathematics",
+    "science",
+    "no-std",
+]
+license = "Apache-2.0 OR MIT"
+repository = "https://github.com/tarcieri/micromath"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = [
+    "--cfg",
+    "docsrs",
+]
+
+[dependencies.num-traits]
+version = "0.2"
+optional = true
+default-features = false
+
+[features]
+quaternion = []
+statistics = []
+vector = []