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/build.rs

@@ -0,0 +1,91 @@
+use std::fs::File;
+use std::io::Write;
+use std::path::{Path, PathBuf};
+use std::{env, ffi::OsStr};
+
+fn main() {
+    let mut target = env::var("TARGET").unwrap();
+
+    // When using a custom target JSON, `$TARGET` contains the path to that JSON file. By
+    // convention, these files are named after the actual target triple, eg.
+    // `thumbv7m-customos-elf.json`, so we extract the file stem here to allow custom target specs.
+    let path = Path::new(&target);
+    if path.extension() == Some(OsStr::new("json")) {
+        target = path
+            .file_stem()
+            .map_or(target.clone(), |stem| stem.to_str().unwrap().to_string());
+    }
+
+    // Put the linker script somewhere the linker can find it
+    let out = &PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    let link_x = include_bytes!("link.x.in");
+    let mut f = if env::var_os("CARGO_FEATURE_DEVICE").is_some() {
+        let mut f = File::create(out.join("link.x")).unwrap();
+
+        f.write_all(link_x).unwrap();
+
+        // *IMPORTANT*: The weak aliases (i.e. `PROVIDED`) must come *after* `EXTERN(__INTERRUPTS)`.
+        // Otherwise the linker will ignore user defined interrupts and always populate the table
+        // with the weak aliases.
+        writeln!(
+            f,
+            r#"
+/* Provides weak aliases (cf. PROVIDED) for device specific interrupt handlers */
+/* This will usually be provided by a device crate generated using svd2rust (see `device.x`) */
+INCLUDE device.x"#
+        )
+        .unwrap();
+        f
+    } else {
+        let mut f = File::create(out.join("link.x")).unwrap();
+        f.write_all(link_x).unwrap();
+        f
+    };
+
+    println!("cargo:rustc-check-cfg=cfg(armv6m)");
+    println!("cargo:rustc-check-cfg=cfg(armv7m)");
+    println!("cargo:rustc-check-cfg=cfg(armv8m)");
+    println!("cargo:rustc-check-cfg=cfg(cortex_m)");
+    println!("cargo:rustc-check-cfg=cfg(has_fpu)");
+
+    let max_int_handlers = if target.starts_with("thumbv6m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv6m");
+        32
+    } else if target.starts_with("thumbv7m-") || target.starts_with("thumbv7em-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+        240
+    } else if target.starts_with("thumbv8m") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        496
+    } else {
+        // Non ARM target. We assume you're just testing the syntax.
+        // This value seems as good as any.
+        240
+    };
+
+    if target.ends_with("-eabihf") {
+        println!("cargo:rustc-cfg=has_fpu");
+    }
+
+    // checking the size of the interrupts portion of the vector table is sub-architecture dependent
+    writeln!(
+        f,
+        r#"
+ASSERT(SIZEOF(.vector_table) <= 0x{:x}, "
+There can't be more than {1} interrupt handlers. This may be a bug in
+your device crate, or you may have registered more than {1} interrupt
+handlers.");
+"#,
+        max_int_handlers * 4 + 0x40,
+        max_int_handlers
+    )
+    .unwrap();
+
+    println!("cargo:rustc-link-search={}", out.display());
+
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=link.x.in");
+}