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/scanner.h

@@ -46,6 +46,9 @@ struct ScanRequest {
 
     ScanCondition condition = ScanCondition::ExactValue;
     int valueSize = 4;              // bytes per value (for unknown scans)
+
+    uint64_t startAddress = 0;      // 0 = no limit (scan all regions)
+    uint64_t endAddress   = 0;      // 0 = no limit (scan all regions)
 };
 
 struct ScanResult {