selection and hover use full-row bg markers instead of indicators

- replace IND_SELECTED/IND_HOVER indicators with M_HOVER(6) M_SELECTED(7) bg markers
- disable caret line highlight and native selection rendering
- re-enable sel rendering only during inline edit
- add drag-select support via nodeClicked shift signals
- use lineRangeNoEol helper to exclude EOL from indicator ranges
- add drag tracking state to editor

src/core.h

@@ -109,6 +109,8 @@ enum Marker : int {
     M_CYCLE     = 3,
     M_ERR       = 4,
     M_STRUCT_BG = 5,
+    M_HOVER     = 6,
+    M_SELECTED  = 7,
 };
 
 // ── Provider interface ──