Add helpful errors and more testing

[tuckermcclure]

Changes made:

• Added julia = "1.9" to Project.toml.
• Loosened Random compat from 1.11.0 to 1.
• Expanded CI to test Julia 1.9, 1.10, 1.11, and 1.12 on Ubuntu, plus 1.12 on Windows/macOS.
• Added friendly fallback errors for YAML/JSON methods when the parser package has not been loaded.
• Added trusted-input warnings to readme.md and module docs.
• Added JSON usage examples to the README.
• Documented to_dict, from_dict, and the parser-specific dictionary functions.
• Updated changelog.md content for the v0.1 surface, leaving it marked unreleased.

Why uppercase filenames? Mostly convention and discoverability. README.md / CHANGELOG.md are what people and tooling tend to scan for first, but lowercase works fine here, and I left them as-is.

Verification:

• julia --project=. -e 'using Pkg; Pkg.resolve(); Pkg.test()' passes.
• Smoke-checked the new YAML/JSON fallback errors.
• Confirmed using PortableStructs alone does not load StaticArrays, YAML, or JSON.