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/basic-toml/tests/invalid-misc.rs

@@ -0,0 +1,48 @@
+use serde_json::Value;
+
+macro_rules! bad {
+    ($toml:expr, $msg:expr) => {
+        match basic_toml::from_str::<Value>($toml) {
+            Ok(s) => panic!("parsed to: {:#?}", s),
+            Err(e) => assert_eq!(e.to_string(), $msg),
+        }
+    };
+}
+
+#[test]
+fn bad() {
+    bad!("a = 01", "invalid number at line 1 column 6");
+    bad!("a = 1__1", "invalid number at line 1 column 5");
+    bad!("a = 1_", "invalid number at line 1 column 5");
+    bad!("''", "expected an equals, found eof at line 1 column 3");
+    bad!("a = 9e99999", "invalid number at line 1 column 5");
+
+    bad!(
+        "a = \"\u{7f}\"",
+        "invalid character in string: `\\u{7f}` at line 1 column 6"
+    );
+    bad!(
+        "a = '\u{7f}'",
+        "invalid character in string: `\\u{7f}` at line 1 column 6"
+    );
+
+    bad!("a = -0x1", "invalid number at line 1 column 5");
+    bad!("a = 0x-1", "invalid number at line 1 column 7");
+
+    // Dotted keys.
+    bad!(
+        "a.b.c = 1
+         a.b = 2
+        ",
+        "duplicate key: `b` for key `a` at line 2 column 12"
+    );
+    bad!(
+        "a = 1
+         a.b = 2",
+        "dotted key attempted to extend non-table type at line 1 column 5"
+    );
+    bad!(
+        "a = {k1 = 1, k1.name = \"joe\"}",
+        "dotted key attempted to extend non-table type at line 1 column 11"
+    );
+}