DatePicker

[colemancda]

Completes the controls tier: a date picker bound to a Foundation Date.

What's new

• DatePicker — DatePicker("Title", selection: $date) with Binding<Date> (Foundation's Date; the core now imports Foundation rather than defining its own date type). The selection crosses the bridge as milliseconds since the Unix epoch — Compose DatePickerState's native currency — so the conversion is a single multiply on each side.
• Interpreter renders a label row with a trailing formatted date (Jan 15, 2000); tapping opens a Material3 DatePickerDialog, and confirming fires the double callback with the picked millis.

Verification

• swift test — DatePicker emission + round-trip test (invoking the change callback updates the emitted millis), 53 total passing
• Emulator (More Controls screen): row shows Jan 1, 2000 from the initial Date; tapping opens the calendar dialog seeded to January 2000 with day 1 selected; picking 15 and confirming updates the row to Jan 15, 2000 — full state round-trip through @State var birthday: Date

Notes

Date-only (UTC midnight semantics, matching the Material date picker); displayedComponents/time selection not modeled. Dialog date formatting is a fixed MMM d, yyyy-style abbreviation, locale-neutral for now.