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

• New Features

  • Added SliverVisibility widget support for conditional sliver visibility with optional replacement and maintain flags

• Examples

  • Added an interactive gallery example showcasing SliverVisibility usage and behavior

• Documentation

  • Added a documentation page describing SliverVisibility properties, behavior, and usage examples

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

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds first-class SliverVisibility support to STAC: new core model and JSON (de)serialization, parser and parser registration, docs and example assets, and exports to expose the new widget type.

Changes

Cohort / File(s)	Summary
Documentation docs/docs.json, docs/widgets/sliver_visibility.mdx	Added navigation entry and new docs page describing SliverVisibility, properties, example, and link to Flutter docs.
Example Gallery examples/stac_gallery/assets/json/home_screen.json, examples/stac_gallery/assets/json/sliver_visibility_example.json	Added gallery list tile and a new example JSON demonstrating a SliverVisibility usage.
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; implemented StacSliverVisibility model with fields (sliver, replacementSliver, visible, maintain* flags); added generated JSON (de)serialization and exported the model.
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	Added StacSliverVisibilityParser, exported it from parsers barrel, and registered the parser in StacService parser list so STAC can parse and build SliverVisibility widgets.

Sequence Diagram(s)

sequenceDiagram
  participant Service as StacService
  participant Parser as StacSliverVisibilityParser
  participant Model as StacSliverVisibility
  participant Builder as FlutterWidgetBuilder

  Service->>Parser: request parser for type "sliverVisibility"
  Parser->>Model: fromJson(json)
  Parser->>Parser: parse nested `sliver` / `replacementSliver` via context
  Parser->>Builder: construct SliverVisibility(sliver, replacementSliver, flags)
  Builder->>Flutter: insert SliverVisibility into widget tree

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• feat: add stacSliverOpacity widget, parser,example and documentation #425: Adds Sliver-related model/parser/docs and modifies same integration points (WidgetType, parser registration, exports) — likely overlapping changes.

Suggested reviewers

• divyanshub024
• Potatomonsta

Poem

🐰 I hopped through JSON, parser, and doc,
I stitched a sliver from code to the block,
Now visible, hidden, or standing in place,
STAC's got a sliver with elegance and grace 🥕✨

🚥 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 new StacSliverVisibility widget with parser, example, and documentation.
Linked Issues check	✅ Passed	The PR fully implements the requirement from issue #418 to create StacSliverVisibility as a StacWidget implementation of Flutter's SliverVisibility.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing StacSliverVisibility and its supporting infrastructure; no unrelated modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[akhil-ge0rge]

Hi @divyanshub024 , @Potatomonsta 👋,
Please take a look. Thanks!

[divyanshub024]

@akhil-ge0rge Let's merge this one as well!!

[akhil-ge0rge]

@akhil-ge0rge Let's merge this one as well!!

@divyanshub024 Done

[divyanshub024]

Thanks a lot for the contribution. Really appreciate the effort 🙌🔥

[akhil-ge0rge]

Thanks a lot for the contribution. Really appreciate the effort 🙌🔥

Thanks a lot! Happy to contribute and really appreciate the support 🙌