feat: tree lines, scanner improvements, themes, tooltips, README overhaul

- Tree line connectors (Unicode box-drawing ├─ └─ │) at arbitrary depth
- Fix editor overwriting tree chars at depth 2+ (applyMarginText Pass 2)
- Scanner: unknown value scan, comparison rescan modes (Changed/Unchanged/Increased/Decreased)
- New Tailwind theme (tw.json), WCAG contrast fixes for warm/mid themes
- Tooltip system (rcxtooltip.h)
- Comprehensive README rewrite with full feature inventory
- New tests for compose tree lines, scanner, tooltips

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

src/themes/defaults/tw.json

@@ -0,0 +1,32 @@
+{
+    "name": "Light",
+    "background": "#e8e8ec",
+    "backgroundAlt": "#dcdce0",
+    "surface": "#d4d4d8",
+    "border": "#b8b8be",
+    "borderFocused": "#6870a0",
+    "button": "#ccccd0",
+    "text": "#1b1b22",
+    "textDim": "#5c5c68",
+    "textMuted": "#84848e",
+    "textFaint": "#a8a8b0",
+    "hover": "#d8d8de",
+    "selected": "#d0d0d8",
+    "selection": "#b4c8e8",
+    "syntaxKeyword": "#4455aa",
+    "syntaxNumber": "#2a7a4c",
+    "syntaxString": "#9a4040",
+    "syntaxComment": "#6a7a6a",
+    "syntaxPreproc": "#787880",
+    "syntaxType": "#2e7a8a",
+    "indHoverSpan": "#5a68a0",
+    "indCmdPill": "#dcdce0",
+    "indDataChanged": "#2a7a4c",
+    "indHeatCold": "#6a6a30",
+    "indHeatWarm": "#a06828",
+    "indHeatHot": "#b83030",
+    "indHintGreen": "#387a44",
+    "markerPtr": "#b83030",
+    "markerCycle": "#9a7010",
+    "markerError": "#e8c8c8"
+}