Sub-cell hit testing API: clickable zones within cells

[witqq]

Summary

Current HitTestResult resolves to cell-level granularity only: { region, row, col }. There is no mechanism for identifying sub-cell interactive elements (expander icons, sort buttons, links, action buttons within a cell).

Current State

• EventTranslator emits cellClick(row, col) — no sub-cell info
• HitRegion has: cell, header, header-sort-icon, header-filter-icon, row-number, row-group-toggle, corner, outside
• Header-level sub-elements (sort/filter icons) ARE supported, but only in column headers
• No API for CellTypeRenderer to declare clickable zones

Proposed API

interface CellHitZone {
  id: string;              // e.g. "expander", "sort-icon", "link", "action-btn"
  x: number;               // relative to cell origin
  y: number;
  width: number;
  height: number;
  cursor?: string;         // CSS cursor on hover
}

interface CellTypeRenderer {
  // ... existing methods ...
  getHitZones?(value: CellValue, cellWidth: number, cellHeight: number, theme: SpreadsheetTheme): CellHitZone[];
}

Events would extend to:

engine.on("cellClick", (row, col, hitZoneId?: string) => { ... });
engine.on("cellHover", (row, col, hitZoneId?: string) => { ... });

Motivation

Consumers (e.g., planeta-worktree2) build custom DimensionCellHitTester classes that subscribe to gridMouseDown/gridMouseMove and manually compute hit zones. A native API would make this unnecessary.

Files to modify

• packages/core/src/types/cell-type-registry.ts — add getHitZones to CellTypeRenderer
• packages/core/src/events/event-translator.ts — resolve sub-cell zones
• packages/core/src/types/interfaces.ts — CellHitZone, extended event types