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/warm.json

@@ -15,8 +15,8 @@
     "selection": "#21213A",
     "syntaxKeyword": "#AA9565",
     "syntaxNumber": "#AAA98C",
-    "syntaxString": "#6B3B21",
-    "syntaxComment": "#464646",
+    "syntaxString": "#C0825A",
+    "syntaxComment": "#8A8878",
     "syntaxPreproc": "#AA9565",
     "syntaxType": "#6B959F",
     "indHoverSpan": "#AA9565",
@@ -25,8 +25,8 @@
     "indHeatCold": "#C4A44A",
     "indHeatWarm": "#AA9565",
     "indHeatHot": "#A05040",
-    "indHintGreen": "#464646",
-    "markerPtr": "#6B3B21",
+    "indHintGreen": "#688A58",
+    "markerPtr": "#B85A42",
     "markerCycle": "#AA9565",
     "markerError": "#3C2121"
 }