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/fixed/Cargo.toml

@@ -0,0 +1,102 @@
+# 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 = "1.83"
+name = "fixed"
+version = "1.29.0"
+build = "build.rs"
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Fixed-point numbers."
+documentation = "https://docs.rs/fixed"
+readme = "README.md"
+keywords = [
+    "dsp",
+    "embedded",
+    "fixed",
+    "math",
+    "numerics",
+]
+categories = [
+    "algorithms",
+    "embedded",
+    "mathematics",
+    "no-std",
+]
+license = "MIT/Apache-2.0"
+repository = "https://gitlab.com/tspiteri/fixed"
+
+[package.metadata.docs.rs]
+features = [
+    "arbitrary",
+    "borsh",
+    "num-traits",
+    "serde",
+    "std",
+]
+
+[features]
+arbitrary = ["dep:arbitrary"]
+az = []
+borsh = ["dep:borsh"]
+f16 = []
+fail-on-warnings = []
+nightly-float = []
+num-traits = ["dep:num-traits"]
+serde = ["dep:serde"]
+serde-str = [
+    "serde",
+    "serde/std",
+    "std",
+]
+std = []
+
+[lib]
+name = "fixed"
+path = "src/lib.rs"
+
+[dependencies.arbitrary]
+version = "1"
+optional = true
+
+[dependencies.az]
+version = "1.2"
+
+[dependencies.borsh]
+version = "1"
+optional = true
+default-features = false
+
+[dependencies.bytemuck]
+version = "1.2"
+
+[dependencies.half]
+version = ">= 1.8, < 3"
+default-features = false
+
+[dependencies.num-traits]
+version = "0.2.18"
+features = ["i128"]
+optional = true
+default-features = false
+
+[dependencies.serde]
+version = "1.0.210"
+optional = true
+default-features = false
+
+[dependencies.typenum]
+version = "1.14"