Add Label and labelStyle

[colemancda]

Label("title", systemImage:) (and the custom title:/icon: builder), plus .labelStyle(.automatic / .titleAndIcon / .titleOnly / .iconOnly).

Shape

Emitted as its own Label node with children [icon, title] rather than desugared to an HStack — that fixed order is what lets labelStyle drop either half. The style rides the same inherited-CompositionLocal mechanism the control styles use (#55): .labelStyle on a container styles every label beneath it, and each label reads LocalLabelStyle to decide whether to show its icon, its title, or both.

Verification

• swift test — 3 new tests, 110 passing: icon-then-title order (the interpreter depends on it), the custom title/icon builder, and — the load-bearing one — that a container carries the style while the labels beneath it carry none of their own, which is what makes inheritance real rather than per-node.
• Emulator (new "Label" screen), screenshot-checked: three icon+title labels (star/trash/envelope) render; .titleOnly shows only the text, .iconOnly shows only the star; and both hearts under one container-level .labelStyle(.iconOnly) drop their titles — inheritance working end to end. The dropped titles are also absent from the accessibility tree, not just hidden.

Scope

Built-in style spellings only — the LabelStyle protocol for custom layouts isn't modeled. .automatic and .titleAndIcon both show both halves.