fix(ios): measure toast leading slots at their rendered width

[rehmatsg]

Summary

Fix narrow iOS toasts that wrap their title and subtitle despite available space. Measurement now reserves the actual rendered leading-slot width (avatar, icon, or progress indicator), matching the live SwiftUI layout. Prepare patch release v0.8.1.

Changes

• Share the actual leading-slot width between measurement and rendering.
• Add native layout and leading-slot regression tests.
• Update package version, changelog, and installation snippets for v0.8.1.

Testing

• Native iOS simulator tests: 2 passed. The avatar layout regression failed before the fix (100pt constrained height versus 60pt natural height) and passes afterward.
• App integration: Vance's complete suite passed with this fix pinned.
• CI runs Dart analysis/tests, Swift compilation, iOS example build, and Android build/tests before merge.

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Summary by cubic

Fixes iOS toast measurement so titles and subtitles no longer wrap inside an undersized toast when there's enough screen space. The width probe now reserves the actual rendered leading-slot width (avatar, icon, or progress indicator) instead of a fixed icon-slot constant, matching the live SwiftUI layout.

• Adds native regression tests covering avatar layout and leading-slot measurement.
• Prepares the v0.8.1 patch release, updating package versions, changelog, and installation snippets.

^{Written for commit d49e647. Summary will update on new commits.}