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/valid/arrays.json

@@ -0,0 +1,34 @@
+{
+    "ints": {
+        "type": "array",
+        "value": [
+            {"type": "integer", "value": "1"},
+            {"type": "integer", "value": "2"},
+            {"type": "integer", "value": "3"}
+        ]
+    },
+    "floats": {
+        "type": "array",
+        "value": [
+            {"type": "float", "value": "1.1"},
+            {"type": "float", "value": "2.1"},
+            {"type": "float", "value": "3.1"}
+        ]
+    },
+    "strings": {
+        "type": "array",
+        "value": [
+            {"type": "string", "value": "a"},
+            {"type": "string", "value": "b"},
+            {"type": "string", "value": "c"}
+        ]
+    },
+    "dates": {
+        "type": "array",
+        "value": [
+            {"type": "datetime", "value": "1987-07-05T17:45:00Z"},
+            {"type": "datetime", "value": "1979-05-27T07:32:00Z"},
+            {"type": "datetime", "value": "2006-06-01T11:00:00Z"}
+        ]
+    }
+}