feat: add CacheStrategy system for fine-grained cache control

[EliaTolin]

Summary

This PR implements the CacheStrategy system as requested in #6, giving developers fine-grained control over how data is retrieved from cache vs remote source.

Changes

• Added CacheStrategy enum with two strategies:
  • cacheFirst (default): Uses cache if available and valid, otherwise fetches from network
  • networkFirst: Always tries network first, falls back to cache (even expired) on failure
• Added strategy parameter to call() method
• Implemented fallback mechanism for networkFirst that uses expired cache data when network fails
• Added comprehensive test coverage (8 new tests)
• Updated README with usage examples, API reference, and FAQ

Usage Example

// Network-first for fresh data with offline fallback
final news = await RemoteCaching.instance.call<News>(
  'latest_news',
  strategy: CacheStrategy.networkFirst,
  remote: () async => await newsService.getLatest(),
  fromJson: (json) => News.fromJson(json as Map<String, dynamic>),
);

Backward Compatibility

• Default strategy remains cacheFirst, preserving existing behavior
• No breaking changes

Test plan

• All existing tests pass
• New tests for cacheFirst strategy behavior
• New tests for networkFirst strategy behavior
• Tests for network failure fallback to cache
• Tests for expired cache fallback
• Flutter analyze passes with no issues

Closes #6

🤖 Generated with Claude Code

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Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.80%. Comparing base (2da88ea) to head (48c5b1d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/remote_caching_impl.dart	83.33%	4 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
- Coverage   90.09%   88.80%   -1.30%     
==========================================
  Files           3        3              
  Lines         101      125      +24     
==========================================
+ Hits           91      111      +20     
- Misses         10       14       +4     

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