Flexible frames: min/max/ideal, fill, and alignment

[colemancda]

Closes the biggest day-to-day layout gap: .frame only supported fixed width/height.

What's new

• Flexible .frame — minWidth/idealWidth/maxWidth/minHeight/idealHeight/maxHeight plus alignment. maxWidth/maxHeight of .infinity fills the available space.
• Fixed .frame now takes alignment: — positions the content within the frame.
• Full Alignment preset set — added .leading/.trailing/.top/.bottom/.topTrailing/.bottomLeading (only .center/.topLeading/.bottomTrailing existed).

.infinity can't cross the bridge as a JSON number, so it's carried as a fillWidth/fillHeight flag; the interpreter maps to fillMaxWidth/fillMaxHeight, finite bounds to widthIn/heightIn, and alignment to wrapContentSize.

Verification

• swift test — 4 new frame tests (default-alignment omission, fill flag, bounds), 61 passing
• Emulator (new Frames screen): fill-width bar, fill + leading/trailing content alignment, a fixed 220×90 box with bottom-trailing content, and a bounded min 120/max 200 box whose text wraps at the cap — all render as SwiftUI would

Incidental

Adds a core-package .gitignore for stray swiftc build outputs (*.swiftmodule, *.dia, …) that had accumulated untracked in the package root.

Deferred

idealWidth/idealHeight are emitted but only inform bounds today (no ideal-size negotiation); per-axis alignment guides beyond the 9 presets.