Give ProgressView a label and a style

[colemancda]

ProgressView worked for the two common cases but had two gaps: it couldn't carry a label, and it conflated two independent things — determinacy and shape.

The conflation

Shape was inferred from whether a value was present: a value meant a linear bar, no value meant a circular spinner. So ProgressView(value:) drawn as a ring was simply impossible to express, as was an indeterminate bar. These are separate axes in SwiftUI, and now here too:

• determinacy — is there a value?
• shape — .progressViewStyle(.linear) / .circular), defaulting to .automatic, which keeps the old value-derived behavior

Labels

ProgressView("Loading…"), ProgressView("Copying", value:), and the builder form ProgressView(value:) { … }. The label resolves as children and renders beneath the indicator. ProgressView is now generic over its label, with EmptyView-constrained initializers so the existing ProgressView() and ProgressView(value:) call sites are untouched (verified — the gallery's existing usages compile unchanged).

Verification

• swift test — 2 new tests, 84 total passing. They cover value normalization against total, both style overrides, and that the label-free forms stay label-free.
• Emulator (ProgressView screen, two new sections), screenshot-checked. The load-bearing one: with progress at 25% the determinate ring shows a quarter arc, and after advancing to 75% the same ring shows a three-quarter arc — determinate-plus-circular tracking a real value, which the old implementation couldn't render at all. The indeterminate linear bar animates alongside it.

Scope

This is the .circular/.linear spelling only — the ProgressViewStyle protocol for custom styles isn't modeled, and neither is currentValueLabel. It is, though, the first style-ish modifier in the codebase, so it sets the shape for the broader style system (.buttonStyle, .listStyle, …) that's still outstanding.