feat(TreeList): customizable expand/collapse indicator icon

[AKnassa]

Closes #4131

What this does

Lets a TreeList choose its own expand/collapse icons instead of the built-in rotating chevron — so a file tree can show a closed folder that becomes an open folder, with file icons on the rows that can't expand.

How

One new tree-level prop, with the shape proposed in the issue:

<TreeList
  items={items}
  renderExpandIcon={({isExpanded, hasChildren}) =>
    hasChildren
      ? <Icon icon={isExpanded ? FolderOpenIcon : FolderIcon} />
      : <Icon icon={DocumentIcon} />
  }
/>

• Called per item with {isExpanded, hasChildren, isDisabled} (exported as TreeListExpandIconState). Leaves get the call too, so file trees can fill the indicator slot — rows with a leaf icon align to the indicator column.
• Custom icons swap per state; the chevron's rotation animation is suppressed for them, per the issue's note (an open folder shouldn't also rotate).
• Returning nothing renderable falls back safely: default chevron for parents, no indicator (and the usual alignment offset) for leaves.
• The toggle's accessibility wiring is untouched regardless of the icon: aria-expanded, localized aria-label, roving tabindex, data-tree-toggle.
• Without the prop, rendering is unchanged.

Per-item overrides stay out, as the issue suggests ("could come later").

How to see it

Storybook → Core/TreeList → Custom Expand Icons (screenshots incoming: light/dark, toggle open/close swap, and the default-chevron stories for before/after comparison).

Proof

• 23 new tests (TreeList suite 48 → 71): red-first for the feature, plus edge cases — disabled parents, children: [], isExpanded on leaves, Enter routing on clickable parents, string/empty returns, deep nesting, render-prop swap on re-render, ArrowLeft collapse. Six deliberate mutations each fail exactly one test.
• Full suite green (core+lab 5090, cli 2178); core/docs/storybook typecheck and lint clean.
• Docs updated (en/zh/dense) and rendering via astryx component TreeList --dense; changeset included.

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Project	Deployment	Actions	Updated (UTC)
astryx	Ready	Preview, Comment	Jul 23, 2026 3:00am

[github-actions]

PR Analysis Report

Modified Components

TreeList

Metric	Before	After	Delta
Bundle Size (ESM)	N/A	N/A	N/A
Lines of Code	N/A	757	-
Complexity	N/A	Very High (80)	-

Bundle Size Summary

Package	Size (ESM)	Size (CJS)	Gzipped
@astryxdesign/core	N/A	4.7KB	0B

Accessibility Audit

Status: No accessibility violations detected.

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