feat: add stacSliverOpacity widget, parser,example and documentation

[akhil-ge0rge]

Description

This PR introduces support for stacSliverOpacity by adding a new widget model and its corresponding parser.

Related Issues

Closes #419

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)
• Code refactor
• Build configuration change
• Documentation
• Chore

Summary by CodeRabbit

• New Features

  • Added support for SliverOpacity widget with full framework integration and configuration options.

• Documentation

  • Added comprehensive documentation for the new SliverOpacity widget, including properties and usage details.

• Examples

  • Added gallery example demonstrating SliverOpacity usage with sample code and visual reference.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR introduces support for the SliverOpacity widget to the Stac framework by adding a new StacSliverOpacity model class, its corresponding parser, registering it in the framework's service layer, extending the WidgetType enum, and providing documentation and gallery examples.

Changes

Cohort / File(s)	Summary
Core Model & JSON Serialization packages/stac_core/lib/widgets/sliver_opacity/stac_sliver_opacity.dart, packages/stac_core/lib/widgets/sliver_opacity/stac_sliver_opacity.g.dart	Introduces JsonSerializable StacSliverOpacity class with required opacity field (0.0–1.0), optional alwaysIncludeSemantics and sliver fields, and generated (de)serialization methods.
Parser Implementation packages/stac/lib/src/parsers/widgets/stac_sliver_opacity/stac_sliver_opacity_parser.dart	Implements StacSliverOpacityParser to convert StacSliverOpacity models into Flutter SliverOpacity widgets, mapping opacity and nested sliver properties.
Parser Registration & Exports packages/stac/lib/src/framework/stac_service.dart, packages/stac/lib/src/parsers/widgets/widgets.dart, packages/stac_core/lib/widgets/widgets.dart	Registers the new parser in StacService and adds public exports in barrel files for accessible imports.
Type System Extension packages/stac_core/lib/foundation/specifications/widget_type.dart	Adds sliverOpacity enum value to WidgetType to support type identification and routing for SliverOpacity widgets.
Documentation & Examples docs/docs.json, docs/widgets/sliver_opacity.mdx, examples/stac_gallery/assets/json/home_screen.json, examples/stac_gallery/assets/json/sliver_opacity_example.json	Adds widget documentation, properties reference, gallery entry, and interactive JSON example demonstrating SliverOpacity with a faded container inside.

Sequence Diagram(s)

sequenceDiagram
    participant App as App/Client
    participant Framework as Stac Framework
    participant Parser as SliverOpacityParser
    participant Flutter as Flutter Engine

    App->>Framework: Load JSON with SliverOpacity
    Framework->>Parser: Identify type: sliverOpacity
    Parser->>Parser: Extract opacity, alwaysIncludeSemantics, sliver
    Parser->>Parser: Parse nested sliver widget
    Parser->>Flutter: Build SliverOpacity widget
    Flutter->>Flutter: Render faded sliver content
    Flutter->>App: Display on screen

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• feat: Introduce stacBadge widget and parser with example JSON #388: Implements parallel SliverAppBar support with identical patterns (model, parser, service registration, WidgetType extension, exports).

Suggested reviewers

• divyanshub024
• Potatomonsta

Poem

🐰 A sliver so pale, now fades with such grace,
Through JSON to Parser, then its rightful place,
Opacity whispers where slivers once gleamed,
The SliverOpacity widget fulfills every dream! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding a SliverOpacity widget with parser, example, and documentation.
Linked Issues check	✅ Passed	The PR implements all coding requirements from issue #419: creates StacSliverOpacity class with proper model, parser integration, and type enum support.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing SliverOpacity support; documentation, examples, and exports are appropriate supporting additions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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