fix(super-editor): repair caret, keyboard, and drag interactions around block SDTs (SD-3237)

[luccas-harbour]

Summary

Repairs hover, click-to-place, Backspace/Delete, and drag-and-drop behaviors around block-level Structured Document Tags (SDTs). The core defect was that block SDTs were "sticky" boundaries: the chrome label was picked up by caret lookups, Backspace/Delete at adjacent paragraph boundaries deleted into protected content (or did nothing useful), and dragging an SDT could snap it back to its source position.

• Caret lookup (DomPositionIndex): excludes structured-content chrome labels from the body position index so hover and click-to-place no longer resolve to the label element when it shares a pm-start/pm-end range with its content.
• Backspace/Delete chain: adds moveIntoBlockSdtBeforeTextBlockStart (backspace chain) and moveIntoBlockSdtAfterTextBlockEnd (delete chain). When the caret is at the textblock boundary adjacent to a block SDT, the caret jumps to the last/first content cursor position inside the SDT instead of deleting into it. Handles nested SDTs, tables inside SDTs, hidden markers (bookmarks, comment ranges, perm markers, hidden field annotations), block atoms, marker-only paragraphs, and empty block SDTs as edge cases.
• Drag-and-drop boundary (internal-node-move): resolveInsertionBoundary now sorts candidate boundaries by distance from the requested target and only uses the bias side as a tie-break. The drop path passes the mapped source start as avoidPos, so a moved block lands at the next nearest valid boundary rather than snapping back to its original slot.
• SDT label CSS (layout-engine/painters/dom/styles.ts): consolidates block and inline label rules into one shared box-model declaration (size, padding, border, background, text color), adds a shared ::before drag-handle indicator (the three-dot vertical grip), and reduces the rule blocks to positioning, border-radius, and the inline display: inline-flex override. Caps block label width at 130px.
• Shared helpers: lifts findFirstTextPosInNode / findLastTextPosInNode out of tableBoundaryNavigation.js into core/commands/helpers/textPositions.js and adds findFirstContentCursorPosInNode / findLastContentCursorPosInNode plus an isZeroWidthMarker predicate covering bookmarks, comment ranges, perm markers, ToC/index/authority entries, passthroughs, and hidden field annotations.

Files

• packages/layout-engine/painters/dom/src/styles.ts + styles.test.ts — shared label box model and drag-handle indicator
• packages/super-editor/src/editors/v1/dom-observer/DomPositionIndex.ts + test — chrome-label exclusion
• packages/super-editor/src/editors/v1/core/commands/helpers/textPositions.js — new shared helper
• packages/super-editor/src/editors/v1/core/commands/moveIntoBlockSdtAtTextBlockBoundary.js + thin re-export shims + ~1.2k lines of unit tests
• packages/super-editor/src/editors/v1/core/commands/{core-command-map.d.ts,core-command-map.test.ts,index.js} — command registration
• packages/super-editor/src/editors/v1/core/extensions/keymap.js + chain test — wires both commands into the Backspace/Delete chains
• packages/super-editor/src/editors/v1/core/presentation-editor/input/internal-node-move.ts + test — distance-sorted boundary resolution with avoidPos
• packages/super-editor/src/editors/v1/extensions/table/tableHelpers/tableBoundaryNavigation.js — switches to shared helpers
• tests/behavior/tests/sdt/sd-3237-sdt-interactions.spec.ts — two new behavior tests for Backspace/Delete into a block SDT containing a table

[cubic-dev-ai]

No issues found across 19 files

_{Re-trigger cubic}

[linear-code]

SD-3237