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/mid.json

@@ -10,8 +10,8 @@
     "textDim": "#505C74",
     "textMuted": "#384258",
     "textFaint": "#2C3448",
-    "hover": "#121720",
-    "selected": "#121720",
+    "hover": "#181E2A",
+    "selected": "#1A2D4A",
     "selection": "#1A2038",
     "syntaxKeyword": "#5688C0",
     "syntaxNumber": "#90B480",