feat: add json-key R source override provider

[eitsupi]

Summary

• add a generic json-key R source override provider with the same fallback and diagnostic behavior as toml-key
• read string versions from dot-separated JSON object paths while keeping raw parse input out of diagnostics
• document the explicit opt-in configuration for reading R.Version from renv.lock, including its snapshot semantics
• update the configuration schema and arf r resolve source metadata
• separate provider metadata, file reading, and common version resolution so adding providers does not grow the main resolution loop

Design

• keep RSourceOverride as the serde/schemars tagged enum
• normalize provider reads as missing, resolved values, or rejected diagnostics
• encode unsupported and file-backed providers with typed reader variants, so invalid file/key combinations are not representable
• preserve provider priority, fallback behavior, diagnostic wording, parse-input redaction, and lazy rig caching

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --locked -- -D warnings
• cargo test --all-targets --all-features --workspace --locked

Closes #321