feat: CnDataMatrix / CnSpreadsheetGrid — inline-edit grid for matrix data

[rubenvdlinde]

What's missing

No spreadsheet-style grid widget. Apps with matrix-shaped data (gradebooks, attendance, planning) need bespoke code.

Consumers waiting

• scholiq: GradebookView (cohort × plan matrix with inline edit, weighted aggregation)
• scholiq: MarkAttendanceView (student × session matrix)
• (future: pipelinq capacity-planning, procest workload allocation)

Sketch

<CnDataMatrix
  :rows="students"
  :cols="sessions"
  :get-cell="(row, col) => ..."
  :set-cell="(row, col, val) => ..."
  :sticky-headers="true"
  :inline-edit="true" />

Effort

LARGE — virtual scrolling, sticky headers, inline-edit with type-aware widgets, paste support.

Cross-reference

ConductionNL/learniq#131.