feat: add stacSliverVisibility widget, parser,example and documentation

[akhil-ge0rge]

Description

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

Related Issues

Closes #418

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

Release Notes

• New Features

  • Added SliverVisibility widget support, enabling conditional visibility control for slivers within scrollable layouts with optional replacement widgets
  • New interactive gallery example demonstrating SliverVisibility functionality and configuration

• Documentation

  • Added comprehensive documentation page covering the SliverVisibility widget, including properties, behavior, and usage examples

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds complete support for Flutter's SliverVisibility widget to the STAC framework, including a new model class, JSON parser, documentation page, example usage, and registration in the framework's parser service.

Changes

Cohort / File(s)	Summary
Documentation docs/docs.json, docs/widgets/sliver_visibility.mdx	Added navigation entry for the new widget and created comprehensive documentation page describing SliverVisibility properties, behavior, and examples.
Example Gallery examples/stac_gallery/assets/json/home_screen.json, examples/stac_gallery/assets/json/sliver_visibility_example.json	Added list tile entry for SliverVisibility demo and created example JSON configuration showing usage with CustomScrollView.
Core Widget Model packages/stac_core/lib/foundation/specifications/widget_type.dart, packages/stac_core/lib/widgets/sliver_visibility/stac_sliver_visibility.dart, packages/stac_core/lib/widgets/sliver_visibility/stac_sliver_visibility.g.dart, packages/stac_core/lib/widgets/widgets.dart	Added sliverVisibility enum value, created StacSliverVisibility model class with properties (sliver, replacementSliver, visible, maintain\* flags), generated JSON serialization, and exported through barrel file.
Parser Implementation packages/stac/lib/src/parsers/widgets/stac_sliver_visibility/stac_sliver_visibility_parser.dart, packages/stac/lib/src/parsers/widgets/widgets.dart, packages/stac/lib/src/framework/stac_service.dart	Implemented StacSliverVisibilityParser to convert STAC models to Flutter SliverVisibility widgets, exported parser, and registered in StacService's parser list.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• feat: Introduce stacBadge widget and parser with example JSON #388: Adds similar new widget type following the same pattern of enum registration, model class creation, parser implementation, and service registration.

Suggested reviewers

• divyanshub024
• Potatomonsta

Poem

🐰 A sliver so sly, now visible or shy,
Our STAC framework grows with wings that fly,
From model to parser, the journey's divine,
SliverVisibility shines in its rightful design! ✨

🚥 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 describes the main change: adding a new SliverVisibility widget with parser, example, and documentation.
Linked Issues check	✅ Passed	The PR fully implements the requirement from issue #418: creates StacSliverVisibility as a StacWidget implementation of Flutter's SliverVisibility with complete supporting infrastructure.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing SliverVisibility support: widget model, parser, enum type, documentation, examples, and necessary exports.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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