[codex] Add reusable binding validator APIs

[ostrowr]

Summary

Make schema validation reusable across both the Python and JavaScript/WebAssembly bindings. The preferred end state is a parsed schema object that callers can reuse for repeated validation instead of reparsing schema text for every check, while Python callers can also validate already-materialized JSON-compatible values directly.

Details

• Add validator_for(...) -> Validator to the Python bindings.
• Expose Validator.is_valid_json(...), Validator.is_valid_value(...), and a short Validator.is_valid(...) alias for JSON-string validation.
• Reject non-finite floats, non-string object keys, and integers outside the supported JSON number range before schema validation.
• Add validator_for(...) -> Validator to the JavaScript/WebAssembly bindings.
• Expose Validator.is_valid(...) for repeated JSON-string validation in JS/Wasm consumers.
• Update the Python typing stub, both binding READMEs, and the Python/Wasm demos.

Testing

• just check
• cargo test -p jsoncompat_py --all-features --locked
• cargo test -p jsoncompat_wasm --all-features --locked
• just python-demo
• wasm-pack build wasm --target web --release
• Suggested manual check: install the built Python wheel and verify validator_for(...) on JSON strings and native Python dictionaries; import the Wasm package and verify validator_for(...).is_valid(...) on generated JSON.