4111 Table border wrong cell

Goal

Fix #4111 so table border toggles target the correct adjacent cell after merges introduce rowSpan / colSpan.

Source Of Truth

• GitHub issue: #4111 Table: the table cell border is removed from the wrong cell
• Type: bug fix
• Expected outcome: after merging cells, toggling top or left borders on adjacent cells updates the correct neighbor cell instead of the wrong path-derived sibling
• Browser surface: yes, but code seam is deterministic package tests

Task Shape

• Tracked issue: GitHub
• Complexity: non-trivial
• Heavyweight: no
• Primary package: packages/table
• Likely root-cause layer: helper query / abstraction seam

Phases

1. Read issue, local learnings, and current helper implementation. Status: done
2. Add failing regression coverage for merged-cell border targeting. Status: done
3. Replace naive adjacent-cell lookup with span-aware lookup. Status: done
4. Run targeted tests + package verification. Status: done
5. Create/update PR and sync issue. Status: pending

Findings

• getLeftTableCell and getTopTableCell still use raw path arithmetic.
• Current setSelectedCellsBorder integration coverage does not cover merged-cell adjacency.
• Recent table-border regression from 2026-03-29 was a different bug in withTableCellSelection; that fix should stay intact.
• Existing merge helper findCellByIndexes already provides the right primitive for span-aware lookup.
• .claude/docs/solutions/patterns/critical-patterns.md does not exist in this repo.
• The red regression was clean once the fixture targeted the real c22 cell: top-border toggle wrote to c13 instead of the spanning c11.
• Shared helper getAdjacentTableCell is the right seam; both getTopTableCell and getLeftTableCell now delegate there.

Verification

• bun test packages/table/src/react/components/TableCellElement/setSelectedCellsBorder.integration.spec.tsx
• bun test packages/table/src/react/components/TableCellElement/setSelectedCellsBorder.integration.spec.tsx packages/table/src/lib/queries/getTopTableCell.spec.tsx
• pnpm --filter @platejs/table test packages/table/src/lib/queries/getTopTableCell.spec.tsx packages/table/src/lib/queries/getSelectedCellsBorders.spec.tsx packages/table/src/react/components/TableCellElement/setSelectedCellsBorder.integration.spec.tsx packages/table/src/react/components/TableCellElement/setSelectedCellsBorder.spec.tsx
• pnpm install
• pnpm turbo build --filter=./packages/table
• pnpm turbo typecheck --filter=./packages/table
• pnpm lint:fix

Risks

• Table selection override can still interfere with path-targeted neighbor writes if the new regression accidentally broadens the selected-cell path set.
• Helper changes affect other query callers, so direct helper behavior should stay stable for non-merged tables.