refactor(superdeck): route image Hero flights through flightShuttleBuilder

[leoafarias]

Intent

Route image Hero flights through Flutter's flightShuttleBuilder path so image transitions share the same Hero integration as text and code, with cross-fade logic owned by ImageHeroFlight.

Impacted package

• superdeck

Changes

• Added the ImageHeroFlight widget whose constructor is directly usable as Flutter's Hero shuttle builder.
• Removed buildFlight callback plumbing and the old image flight function.
• Preserved endpoint image reuse for resolved, decoded, and scaled image widgets, including their cross-fade when a shared URI has distinct pre-rendered endpoints.
• Updated image Hero transition coverage for same-URI, cross-fade, and pre-built endpoint cases.
• Documented continuous same-source flights and different-source cross-fades in the Markdown and deck-options references.
• Updated the layout showcase image-treatment example to demonstrate a true full-bleed image with no margin or padding, and unified card styling, accent colors, footer contrast, and transition timing.

Verification

• fvm dart run melos run build_runner:build
• fvm dart run melos run analyze:dart
• fvm flutter test test/src/ui/widgets/hero_element_test.dart test/src/ui/widgets/text_hero_flight_test.dart test/src/markdown/hero_transition_test.dart --reporter expanded (from packages/superdeck)
• fvm dart analyze packages/superdeck
• fvm flutter test packages/superdeck/test/src/ui/widgets
• fvm flutter test packages/superdeck/test/src/markdown/hero_transition_test.dart --plain-name 'image Hero interpolates its constraint-derived block size'
• fvm flutter test packages/superdeck/test/src/markdown/hero_transition_test.dart --plain-name 'image shuttle uses overlay bounds over stale block dimensions'
• fvm flutter test packages/superdeck/test/src/markdown/hero_transition_test.dart --plain-name 'different image sources blend in both navigation directions'
• Demo: fvm dart run tool/build_layout_showcase.dart and fvm flutter run -t lib/layout_showcase_main.dart -d macos; manually stepped across slides 3↔4 after a hot restart with no runtime exceptions.

fvm dart run melos run analyze completed Dart analysis but could not run DCM because the local DCM installation is not licensed.

[github-actions]

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