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/esp-hal-procmacros/src/interrupt.rs

@@ -0,0 +1,47 @@
+use proc_macro::TokenStream;
+use syn::{parse::Error, spanned::Spanned, AttrStyle, Attribute};
+
+pub(crate) enum WhiteListCaller {
+    Interrupt,
+}
+
+pub(crate) fn check_attr_whitelist(
+    attrs: &[Attribute],
+    caller: WhiteListCaller,
+) -> Result<(), TokenStream> {
+    let whitelist = &[
+        "doc",
+        "link_section",
+        "cfg",
+        "allow",
+        "warn",
+        "deny",
+        "forbid",
+        "cold",
+        "ram",
+        "inline",
+    ];
+
+    'o: for attr in attrs {
+        for val in whitelist {
+            if eq(attr, val) {
+                continue 'o;
+            }
+        }
+
+        let err_str = match caller {
+            WhiteListCaller::Interrupt => {
+                "this attribute is not allowed on an interrupt handler controlled by esp-hal"
+            }
+        };
+
+        return Err(Error::new(attr.span(), err_str).to_compile_error().into());
+    }
+
+    Ok(())
+}
+
+/// Returns `true` if `attr.path` matches `name`
+fn eq(attr: &Attribute, name: &str) -> bool {
+    attr.style == AttrStyle::Outer && attr.path().is_ident(name)
+}