✅ test(parser): add insta snapshot coverage

[zrr1999]

Summary

• Enable insta as a workspace testing dependency.
• Add insta to sporec-parser dev-dependencies.
• Add a parser AST snapshot test that captures core language surface syntax and commits the generated baseline snapshot.

Validation

• INSTA_UPDATE=always cargo test -p sporec-parser core_surface_ast_snapshot
• cargo fmt --all -- --check
• cargo test -p sporec-parser
• cargo clippy -p sporec-parser --all-targets -- -D warnings
• GitHub Actions passed Rust checks on head 97db8c7: test, msrv, prek, and wheel jobs.

Notes

• The previous PR metadata used a non-template Impact heading and a [codex] title prefix, which caused the PR-only CI checks to fail.
• No code changes were needed for this CI fix.

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