refactor: Enhance the Positioned widget with new constructors

[divyanshub024]

Description

Enhance the Positioned widget with new constructors

• Clarify the functionality of the Stac Positioned widget and its relation to Flutter's Positioned widget.
• Add detailed explanations of positioning constraints and properties, including nullability.
• Introduce new named constructors: fromRect and fromRelativeRect, with usage examples.
• Update onboarding screen example to utilize StacPositioned.fill for better layout management.
• Change import from 'flutter/cupertino.dart' to 'flutter/material.dart' in the positioned parser for consistency.

Related Issues

Closes #

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 four new constructors to Positioned: fill() for full stack coverage, fromRect() for rect-based positioning, fromRelativeRect() for relative offsets, and directional() for text-direction-aware layouts.

• Documentation

  • Enhanced Positioned widget documentation with positioning constraint rules and comprehensive constructor descriptions.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR introduces four new convenience constructors to the StacPositioned widget (fromRect, fromRelativeRect, fill, directional), expands documentation to clarify positioning constraints, updates an example to use the new fill constructor, and changes a library import from cupertino to material.

Changes

Cohort / File(s)	Summary
Documentation Overhaul docs/widgets/positioned.mdx	Comprehensively rewrote Positioned documentation with expanded descriptions, new positioning constraints section, optional property markers, and documented all four new Dart-only constructors with examples.
Core Constructor Additions packages/stac_core/lib/widgets/positioned/stac_positioned.dart	Added four public constructors/factories: fromRect() mapping rect properties, fromRelativeRect() with explicit edge values, fill() with 0.0 defaults for full coverage, and directional() computing left/right from text direction.
Example Modernization examples/movie_app/stac/onboarding_screen.dart	Refactored explicit positioning (left: 0, right: 0, bottom: 0) to use the new StacPositioned.fill() constructor.
Library Import Update packages/stac/lib/src/parsers/widgets/stac_positioned/stac_positioned_parser.dart	Changed import from Flutter cupertino to material library for widget theming context.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Four paths now pave the way to stack,
Fill and rect, directional and back—
Where bunnies hop with width and height,
Positioned widgets, oh what a delight! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: adding new constructors (fromRect, fromRelativeRect, fill, directional) to the StacPositioned widget, which is the primary code contribution alongside documentation updates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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