refactor(mosaic): make the fullscreen profile a real dialog variant

[alexcarpenter]

Description

Stacked on #10176. Restructures the fullscreen profile dialog without changing how it looks or the variant='fullscreen' API.

• Width belongs to the Profile. The fullscreen Profile sizes itself with inline-size: 100cqi, and the popup keeps fit-content like every other variant. The popup's fullscreen width: 100% rule is removed.
• fullscreen is a real dialog variant. It has its own entry in each variant table (variants, viewportVariants, trackVariants, backdropMotion, popupMotion, closeInsets), sharing the profile values through constants. Its track entry removes the gap around the popup. surfaceOf() and the extra trackVariants.fullscreen patch are gone. The places that care whether a dialog holds a profile use holdsProfile(variant).
• One dismiss decision in Profile. Root resolves 'back' | 'corner' | 'none' once and shares it through context, replacing the separate fullscreen && !compact and navLayout === 'column' checks.
• layoutFullscreen overrides layoutInDialog with only borderRadius: 0 and boxShadow: 'none', instead of copying it.
• Class-probe tests removed for the fullscreen track and Profile width; the behavior tests stay.

One behavior change: a fullscreen dialog with role="alertdialog" no longer shows the Back button, matching how a profile alert dialog shows no corner close.

Not included: safe-area insets. On notched devices with viewport-fit=cover, the fullscreen and compact profile dialogs place controls under the notch and home indicator. That also affects the plain profile dialog on main, so it will be a separate PR.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

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