Add DisclosureGroup

[colemancda]

DisclosureGroup — a label that expands to reveal its content. Supports a plain title or a view label, and either self-managed or bound expansion.

Shape

Emitted as one node whose children are [label..., content...], with a labelCount prop marking where the header ends and the body begins. Expansion has two modes:

• self-managed (no binding) — the interpreter remembers the open/closed state locally; nothing crosses the bridge
• bound (isExpanded:) — the header tap round-trips through a callback so Swift stays the source of truth, exactly like a Toggle

The interpreter draws a tappable header (label + a chevron that flips up/down) and shows the content only while expanded.

Verification

• swift test — 3 new tests, 115 passing: the label/content boundary via labelCount, that an unbound group emits no expansion state (the interpreter owns it), the bound round-trip (onToggle flips isExpanded), and a Label view as the header.
• Emulator (new "DisclosureGroup" screen), screenshot-checked:
  • self-managed: tapping "What's included" revealed its three bullets and flipped the chevron up
  • bound: tapping an external "Expand from outside" button opened the "Advanced settings" group — the binding drives expansion, not just header taps — and the button flipped to "Collapse from outside"
  • the third group renders a Label header (star + "Notifications") and stays collapsed with a down-chevron

Scope

DisclosureGroupStyle for custom presentation isn't modeled; the built-in header (label + chevron, content indented) is the only layout.