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/unicode-ident/tests/compare.rs

@@ -0,0 +1,68 @@
+#![allow(
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
+)]
+
+mod fst;
+mod roaring;
+mod trie;
+
+#[test]
+fn compare_all_implementations() {
+    let xid_start_fst = fst::xid_start_fst();
+    let xid_continue_fst = fst::xid_continue_fst();
+    let xid_start_roaring = roaring::xid_start_bitmap();
+    let xid_continue_roaring = roaring::xid_continue_bitmap();
+
+    for ch in '\0'..=char::MAX {
+        let thought_to_be_start = unicode_ident::is_xid_start(ch);
+        let thought_to_be_continue = unicode_ident::is_xid_continue(ch);
+
+        // unicode-xid
+        assert_eq!(
+            thought_to_be_start,
+            unicode_xid::UnicodeXID::is_xid_start(ch),
+            "{ch:?}",
+        );
+        assert_eq!(
+            thought_to_be_continue,
+            unicode_xid::UnicodeXID::is_xid_continue(ch),
+            "{ch:?}",
+        );
+
+        // ucd-trie
+        assert_eq!(
+            thought_to_be_start,
+            trie::XID_START.contains_char(ch),
+            "{ch:?}",
+        );
+        assert_eq!(
+            thought_to_be_continue,
+            trie::XID_CONTINUE.contains_char(ch),
+            "{ch:?}",
+        );
+
+        // fst
+        assert_eq!(
+            thought_to_be_start,
+            xid_start_fst.contains((ch as u32).to_be_bytes()),
+            "{ch:?}",
+        );
+        assert_eq!(
+            thought_to_be_continue,
+            xid_continue_fst.contains((ch as u32).to_be_bytes()),
+            "{ch:?}",
+        );
+
+        // roaring
+        assert_eq!(
+            thought_to_be_start,
+            xid_start_roaring.contains(ch as u32),
+            "{ch:?}",
+        );
+        assert_eq!(
+            thought_to_be_continue,
+            xid_continue_roaring.contains(ch as u32),
+            "{ch:?}",
+        );
+    }
+}