Array element offset display, fold arrow UX, type picker popup, and provider cleanup

- Show relative hex offset on array element separators ([N] +0x...)
- Dim fold arrows and add hover highlight for better visibility
- Extend fold/chevron click areas for easier interaction
- Add type picker popup for array element type and pointer target editing
- Remove process_provider.h in favor of plugin-based provider system
- Expand compose/format to handle struct-of-array type names and widths

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/examples/demo.rcx

@@ -317,27 +317,27 @@
             "strLen": 64
         },
         {
-            "arrayLen": 1,
+            "arrayLen": 4,
             "collapsed": false,
-            "elementKind": "UInt8",
+            "elementKind": "Float",
             "id": "27",
-            "kind": "Hex64",
-            "name": "field_70",
+            "kind": "Array",
+            "name": "scores",
             "offset": 112,
             "parentId": "1",
             "refId": "0",
             "strLen": 64
         },
         {
-            "arrayLen": 1,
+            "arrayLen": 2,
             "collapsed": false,
-            "elementKind": "UInt8",
+            "elementKind": "Struct",
             "id": "28",
-            "kind": "Hex64",
-            "name": "field_78",
-            "offset": 120,
+            "kind": "Array",
+            "name": "materials",
+            "offset": 128,
             "parentId": "1",
-            "refId": "0",
+            "refId": "20",
             "strLen": 64
         }
     ]