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/panic-probe/Cargo.toml

@@ -0,0 +1,61 @@
+# 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 = "panic-probe"
+version = "0.3.2"
+authors = ["The Knurling Authors"]
+build = "build.rs"
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Panic handler that exits `probe-run` with an error code"
+readme = "README.md"
+keywords = [
+    "knurling",
+    "panic-impl",
+    "defmt",
+    "probe-run",
+]
+categories = [
+    "embedded",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/knurling-rs/defmt"
+
+[package.metadata.docs.rs]
+default-target = "thumbv7m-none-eabi"
+
+[lib]
+name = "panic_probe"
+path = "src/lib.rs"
+
+[dependencies.cortex-m]
+version = "0.7"
+
+[dependencies.defmt]
+version = "0.3"
+optional = true
+
+[dependencies.rtt-target]
+version = "0.5"
+optional = true
+
+[features]
+defmt-error = []
+print-defmt = [
+    "defmt",
+    "defmt-error",
+]
+print-rtt = ["rtt-target"]