feat(transition): omit fixed back-button clone when requested

[rdlabo]

Summary

• Add an optional transition predicate so themes can omit the fixed back-button clone without changing the page transition.
• Keep the default behavior for existing consumers, including iOS 26.
• Include the existing coordinate-refresh fix from this branch, which ios27 already pins.

Verification

• npm test (16 passed)
• npm run build
• npm run lint
• npm run fmt:check

Consumer: ionic-theme-ios27 foldable mode.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.