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/dotted-keys.json

@@ -0,0 +1,34 @@
+{
+  "a": {
+    "b": {
+      "type": "integer",
+      "value": "123"
+    }
+  },
+  "table": {
+    "a": {
+      "b": {
+        "c": {
+          "type": "integer",
+          "value": "1"
+        },
+        "d": {
+          "type": "integer",
+          "value": "2"
+        }
+      }
+    },
+    "in": {
+      "type": {
+        "color": {
+          "type": "string",
+          "value": "blue"
+        },
+        "name": {
+          "type": "string",
+          "value": "cat"
+        }
+      }
+    }
+  }
+}