add icon field to manifest

[dlipicar]

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[Copilot]

Pull request overview

This pull request adds an icon field to the manifest structure of LGX packages, allowing packages to specify a relative path to an icon file bundled within the package. The implementation follows the same pattern as other metadata fields like description, author, and category.

Changes:

• Added icon field as a required string field to the manifest schema (can be empty string)
• Added C API functions lgx_get_icon() and lgx_set_icon() for accessing icon metadata
• Updated documentation to reflect the new field in package specifications

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/core/manifest.h	Added icon field declaration to Manifest class
src/core/manifest.cpp	Added icon field initialization, JSON parsing validation, and serialization
src/core/package.cpp	Initialized icon field to empty string in package creation
src/lgx.h	Added C API declarations for lgx_get_icon() and lgx_set_icon()
src/lib.cpp	Added C API implementations with caching and error handling
tests/test_manifest.cpp	Added comprehensive tests for icon field parsing, missing icon validation, and roundtrip serialization
tests/test_lib.cpp	Added API tests for icon getter/setter functions and null handling
docs/spec.md	Updated manifest schema documentation and default values
docs/project.md	Updated C API documentation

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