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-macros/src/construct/symbol.rs

@@ -0,0 +1,78 @@
+use std::fmt::Write;
+
+use crate::cargo;
+
+pub(crate) fn mangled(defmt_tag: &str, data: &str) -> String {
+    Symbol::new(defmt_tag, data).mangle()
+}
+
+struct Symbol<'a> {
+    /// Name of the Cargo package in which the symbol is being instantiated. Used for avoiding
+    /// symbol name collisions.
+    package: String,
+
+    /// Unique identifier that disambiguates otherwise equivalent invocations in the same crate.
+    disambiguator: u64,
+
+    /// Symbol categorization. Known values:
+    /// * `defmt_prim` for primitive formatting strings that are placed at the start of the `.defmt`
+    ///   section.
+    /// * `defmt_fmt`, `defmt_str` for interned format strings and string literals.
+    /// * `defmt_println` for logging messages that are always displayed.
+    /// * `defmt_trace`, `defmt_debug`, `defmt_info`, `defmt_warn`, `defmt_error` for logging
+    ///   messages used at the different log levels.
+    /// * `defmt_bitflags` indicates that a format string was generated by a `defmt::bitflags!`
+    ///   invocation, and that the decoder should look up possible flags in the binary.
+    ///   The data string is of the format `NAME@REPR#NUM`, where `NAME` is the name of the bitflags
+    ///   struct, `REPR` is the raw integer type representing the bitflags value (and also the
+    ///   wire format), and `NUM` is the number of defined bitflag values.
+    /// * `defmt_bitflags_value` marks a `static` that holds the value of a bitflags `const`, its
+    ///   data field is `STRUCT_NAME::FLAG_NAME`.
+    /// * Anything starting with `defmt_` is reserved for use by defmt, other prefixes are free for
+    ///   use by third-party apps (but they all should use a prefix!).
+    tag: String,
+
+    /// Symbol data for use by the host tooling. Interpretation depends on `tag`.
+    data: &'a str,
+
+    /// Crate name obtained via CARGO_CRATE_NAME (added since a Cargo package can contain many crates).
+    crate_name: String,
+}
+
+impl<'a> Symbol<'a> {
+    fn new(tag: &'a str, data: &'a str) -> Self {
+        Self {
+            // `CARGO_PKG_NAME` is set to the invoking package's name.
+            package: cargo::package_name(),
+            disambiguator: super::crate_local_disambiguator(),
+            tag: format!("defmt_{tag}"),
+            data,
+            crate_name: cargo::crate_name(),
+        }
+    }
+
+    fn mangle(&self) -> String {
+        format!(
+            r#"{{"package":"{}","tag":"{}","data":"{}","disambiguator":"{}","crate_name":"{}"}}"#,
+            json_escape(&self.package),
+            json_escape(&self.tag),
+            json_escape(self.data),
+            self.disambiguator,
+            json_escape(&self.crate_name),
+        )
+    }
+}
+
+fn json_escape(string: &str) -> String {
+    let mut escaped = String::new();
+    for c in string.chars() {
+        match c {
+            '\\' => escaped.push_str("\\\\"),
+            '\"' => escaped.push_str("\\\""),
+            '\n' => escaped.push_str("\\n"),
+            c if c.is_control() || c == '@' => write!(escaped, "\\u{:04x}", c as u32).unwrap(),
+            c => escaped.push(c),
+        }
+    }
+    escaped
+}