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/defmt/Cargo.toml

@@ -0,0 +1,89 @@
+# 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"
+name = "defmt"
+version = "1.0.1"
+authors = ["The Knurling-rs developers"]
+build = "build.rs"
+links = "defmt"
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "A highly efficient logging framework that targets resource-constrained devices, like microcontrollers"
+homepage = "https://knurling.ferrous-systems.com/"
+readme = "README.md"
+keywords = [
+    "knurling",
+    "logging",
+    "logger",
+    "formatting",
+    "formatter",
+]
+categories = [
+    "embedded",
+    "no-std",
+    "development-tools::debugging",
+    "value-formatting",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/knurling-rs/defmt"
+
+[package.metadata.docs.rs]
+features = ["alloc"]
+rustdoc-args = ["--cfg=docsrs"]
+targets = [
+    "thumbv6m-none-eabi",
+    "thumbv7em-none-eabihf",
+]
+
+[features]
+alloc = []
+avoid-default-panic = []
+encoding-raw = []
+encoding-rzcobs = []
+ip_in_core = []
+unstable-test = ["defmt-macros/unstable-test"]
+
+[lib]
+name = "defmt"
+path = "src/lib.rs"
+
+[[test]]
+name = "basic_usage"
+path = "tests/basic_usage.rs"
+
+[[test]]
+name = "derive-bounds"
+path = "tests/derive-bounds.rs"
+
+[[test]]
+name = "encode"
+path = "tests/encode.rs"
+
+[[test]]
+name = "ui"
+path = "tests/ui.rs"
+
+[dependencies.bitflags]
+version = "1"
+
+[dependencies.defmt-macros]
+version = "=1.0.1"
+
+[dev-dependencies.rustc_version]
+version = "0.4"
+
+[dev-dependencies.trybuild]
+version = "1"