Add `field.matrix()` for grid-based survey questions

[akladekouassi]

Summary

Add field.matrix(label) — a grid where each row is an item and each column is a rating / option. The classic survey "rate each item on this scale" pattern.

Why this would help

• Surveys, questionnaires, multi-criteria evaluations, peer reviews.
• Replicating this with N separate radio fields is verbose and brittle to add/remove rows.
• A composite field stores Record<rowKey, columnValue> so downstream code consumes a single shape.

Proposed API

field.matrix('Rate each')
  .rows([
    { key: 'design', label: 'Design' },
    { key: 'usability', label: 'Usability' },
    { key: 'performance', label: 'Performance' },
  ])
  .columns([1, 2, 3, 4, 5])
  .inputType('radio');               // 'radio' | 'checkbox'

Suggested implementation areas

• src/core/field-descriptors/matrix/MatrixFieldBuilder.ts (new)
• src/core/field-descriptors/field.ts
• src/renderers/web/ — semantic table with proper headers and aria-labels
• src/renderers/native/ — vertical row stacks with column-as-segmented-control

Acceptance Criteria

• Stores Record<rowKey, columnValue> (or arrays for inputType('checkbox'))
• Required validation triggers when any row is unanswered
• Semantic markup on web (proper <table> + headers)
• Tests for the radio and checkbox modes

[github-actions]

Mirrored to monorepo issue: https://github.com/runilib/runilib/issues/189