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/ufmt-write/.cargo-checksum.json

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+{"files":{".cargo_vcs_info.json":"352649b6eef8336913ec817c47a7b937c19e822f51b3dca9d14a1effae1eb79d","Cargo.toml":"c8cb5b907495d7c398b900ae506da06148733ec73c70deb5e72dec422b48dc68","Cargo.toml.orig":"48e353b184cb9423968e707e0a6f0404709fe087667d9d3051f81c8a52a7d8f1","src/lib.rs":"0efe6a0124ca4abde04084f1d90e6055d49510717cd6fcaf09fba3d18d5a4b07"},"package":"e87a2ed6b42ec5e28cc3b94c09982969e9227600b2e3dcbc1db927a84c06bd69"}

vendor/ufmt-write/.cargo_vcs_info.json

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+{
+  "git": {
+    "sha1": "f59b61d7ed3dd9d98846f9557f15b556c2959170"
+  }
+}

vendor/ufmt-write/Cargo.toml

@@ -0,0 +1,25 @@
+# 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 = "ufmt-write"
+version = "0.1.0"
+authors = ["Jorge Aparicio <jorge@japaric.io>"]
+description = "`μfmt`'s `uWrite` trait"
+keywords = ["Debug", "Display", "Write", "format"]
+categories = ["embedded", "no-std"]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/japaric/ufmt"
+
+[features]
+std = []

vendor/ufmt-write/src/lib.rs

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+//! `μfmt`'s `uWrite` trait
+
+#![cfg_attr(not(feature = "std"), no_std)]
+#![deny(missing_docs)]
+#![deny(rust_2018_compatibility)]
+#![deny(rust_2018_idioms)]
+#![deny(warnings)]
+
+#[cfg(feature = "std")]
+use core::convert::Infallible;
+
+#[allow(deprecated)]
+unsafe fn uninitialized<T>() -> T {
+    core::mem::uninitialized()
+}
+
+/// A collection of methods that are required / used to format a message into a stream.
+#[allow(non_camel_case_types)]
+pub trait uWrite {
+    /// The error associated to this writer
+    type Error;
+
+    /// Writes a string slice into this writer, returning whether the write succeeded.
+    ///
+    /// This method can only succeed if the entire string slice was successfully written, and this
+    /// method will not return until all data has been written or an error occurs.
+    fn write_str(&mut self, s: &str) -> Result<(), Self::Error>;
+
+    /// Writes a [`char`] into this writer, returning whether the write succeeded.
+    ///
+    /// A single [`char`] may be encoded as more than one byte. This method can only succeed if the
+    /// entire byte sequence was successfully written, and this method will not return until all
+    /// data has been written or an error occurs.
+    fn write_char(&mut self, c: char) -> Result<(), Self::Error> {
+        let mut buf: [u8; 4] = unsafe { uninitialized() };
+        self.write_str(c.encode_utf8(&mut buf))
+    }
+}
+
+#[cfg(feature = "std")]
+impl uWrite for String {
+    type Error = Infallible;
+
+    fn write_str(&mut self, s: &str) -> Result<(), Infallible> {
+        self.push_str(s);
+        Ok(())
+    }
+}