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/cortex-m-rt/Cargo.toml

@@ -0,0 +1,155 @@
+# 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 = "1.61"
+name = "cortex-m-rt"
+version = "0.7.5"
+authors = [
+    "The Cortex-M Team <cortex-m@teams.rust-embedded.org>",
+    "Jorge Aparicio <jorge@japaric.io>",
+    "Hideki Sekine <sekineh@me.com>",
+]
+build = "build.rs"
+links = "cortex-m-rt"
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Minimal runtime / startup for Cortex-M microcontrollers"
+documentation = "https://docs.rs/cortex-m-rt/"
+readme = "README.md"
+keywords = [
+    "arm",
+    "cortex-m",
+    "runtime",
+    "startup",
+]
+categories = [
+    "embedded",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/rust-embedded/cortex-m"
+
+[package.metadata.docs.rs]
+features = ["device"]
+
+[lib]
+name = "cortex_m_rt"
+path = "src/lib.rs"
+
+[[example]]
+name = "alignment"
+path = "examples/alignment.rs"
+
+[[example]]
+name = "cfg-static"
+path = "examples/cfg-static.rs"
+
+[[example]]
+name = "data_overflow"
+path = "examples/data_overflow.rs"
+
+[[example]]
+name = "device"
+path = "examples/device.rs"
+required-features = ["device"]
+
+[[example]]
+name = "divergent-default-handler"
+path = "examples/divergent-default-handler.rs"
+
+[[example]]
+name = "divergent-exception"
+path = "examples/divergent-exception.rs"
+
+[[example]]
+name = "entry-static"
+path = "examples/entry-static.rs"
+
+[[example]]
+name = "hard-fault-trampoline"
+path = "examples/hard-fault-trampoline.rs"
+
+[[example]]
+name = "main"
+path = "examples/main.rs"
+
+[[example]]
+name = "minimal"
+path = "examples/minimal.rs"
+
+[[example]]
+name = "override-exception"
+path = "examples/override-exception.rs"
+
+[[example]]
+name = "pre_init"
+path = "examples/pre_init.rs"
+
+[[example]]
+name = "qemu"
+path = "examples/qemu.rs"
+
+[[example]]
+name = "state"
+path = "examples/state.rs"
+
+[[example]]
+name = "unsafe-default-handler"
+path = "examples/unsafe-default-handler.rs"
+
+[[example]]
+name = "unsafe-entry"
+path = "examples/unsafe-entry.rs"
+
+[[example]]
+name = "unsafe-exception"
+path = "examples/unsafe-exception.rs"
+
+[[example]]
+name = "unsafe-hard-fault"
+path = "examples/unsafe-hard-fault.rs"
+
+[[example]]
+name = "unsafety"
+path = "examples/unsafety.rs"
+
+[[example]]
+name = "warnings"
+path = "examples/warnings.rs"
+required-features = ["device"]
+
+[[test]]
+name = "compiletest"
+path = "tests/compiletest.rs"
+required-features = ["device"]
+
+[dependencies.cortex-m-rt-macros]
+version = "=0.7.5"
+
+[dev-dependencies.cortex-m]
+version = "0.7.4"
+
+[dev-dependencies.panic-halt]
+version = "0.2.0"
+
+[features]
+device = []
+paint-stack = []
+set-sp = []
+set-vtor = []
+zero-init-ram = []
+
+[target.'cfg(not(target_os = "none"))'.dev-dependencies.compiletest_rs]
+version = "0.11"