[koehlma]

Add compiled regression tests and documentation for the Rust builder plugin's existing Default implementation. The tests cover empty records, fields marked #[default], and optional generic fields whose type argument does not implement Default.

Rationale

Records with a zero-argument new() already implement Default by calling Self::new(), but this behavior was undocumented and had no dedicated regression coverage. The tests verify the generated code compiles and that default() and new() produce the same values and JSON representation.

Validation

• cargo test -p sidex-serde-tests -p sidex-gen-rs --locked: 15 tests passed.
• cargo clippy -p sidex-serde-tests -p sidex-gen-rs --all-targets --all-features --locked -- -D warnings: passed.
• cargo +nightly-2026-07-14 fmt --all -- --check: passed.
• uv run --project tests/py --locked pytest -q tests/py/test_json_serde.py tests/py/test_mypy.py: 59 tests passed.

^{Created on behalf of @koehlma by Codex / GPT-6-astra (high) · Task}