Handle boolean merge features

[Medowhill]

Summary

• Detect boolean-valued merge configuration parameters from translations.json.
• Emit a single Cargo feature for boolean parameters, using the parameter name for the true case.
• Lower false-side Rust cfgs to not(feature = "...") while preserving PARAM_value features for non-boolean parameters.
• Add in-memory regression coverage for feature union generation and cfg lowering.

Testing

• cargo fmt
• cargo test -p merger
• cargo clippy --workspace --all-targets
• cargo test --workspace