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/float-cmp/Cargo.toml

@@ -0,0 +1,39 @@
+# 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 believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "float-cmp"
+version = "0.9.0"
+authors = ["Mike Dilger <mike@mikedilger.com>"]
+description = "Floating point approximate comparison traits"
+documentation = "https://docs.rs/float-cmp"
+readme = "README.md"
+keywords = ["float", "comparison", "fuzzy", "approximate", "no_std"]
+license = "MIT"
+repository = "https://github.com/mikedilger/float-cmp"
+
+[lib]
+name = "float_cmp"
+path = "src/lib.rs"
+test = true
+doctest = true
+doc = true
+[dependencies.num-traits]
+version = "0.2.1"
+optional = true
+default-features = false
+
+[features]
+default = ["ratio"]
+ratio = ["num-traits"]
+std = []