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/string-escapes.json

@@ -0,0 +1,58 @@
+{
+    "backspace": {
+        "type": "string",
+        "value": "This string has a \u0008 backspace character."
+    },
+    "tab": {
+        "type": "string",
+        "value": "This string has a \u0009 tab character."
+    },
+    "newline": {
+        "type": "string",
+        "value": "This string has a \u000A new line character."
+    },
+    "formfeed": {
+        "type": "string",
+        "value": "This string has a \u000C form feed character."
+    },
+    "carriage": {
+        "type": "string",
+        "value": "This string has a \u000D carriage return character."
+    },
+    "quote": {
+        "type": "string",
+        "value": "This string has a \u0022 quote character."
+    },
+    "slash": {
+        "type": "string",
+        "value": "This string has a \u002F slash character."
+    },
+    "backslash": {
+        "type": "string",
+        "value": "This string has a \u005C backslash character."
+    },
+    "notunicode1": {
+        "type": "string",
+        "value": "This string does not have a unicode \\u escape."
+    },
+    "notunicode2": {
+        "type": "string",
+        "value": "This string does not have a unicode \u005Cu escape."
+    },
+    "notunicode3": {
+        "type": "string",
+        "value": "This string does not have a unicode \\u0075 escape."
+    },
+    "notunicode4": {
+        "type": "string",
+        "value": "This string does not have a unicode \\\u0075 escape."
+    },
+    "delete": {
+        "type": "string",
+        "value": "This string has a \u007f delete control code."
+    },
+    "unitseparator": {
+        "type": "string",
+        "value": "This string has a \u001f unit separator control code."
+    }
+}