refactor: move unit tests into <module>_tests.rs files

[tupe12334]

Summary

Follow-up to #209 (review request: "extract the tests to <x>_tests.rs files"). Originally stacked on #209; now rebased onto main after #209 merged.

• Every inline #[cfg(test)] mod tests { ... } in core/src/ now lives in a sibling <module>_tests.rs, declared as #[cfg(test)] #[path = "<module>_tests.rs"] mod tests;. Test names, use super::* access to private items, and the #[allow(...)] attributes are unchanged.
• The proptest_tests modules in cel_eval and flow go to <module>_proptest_tests.rs. proptest-regressions/cel_eval.txt is renamed to cel_eval_proptest_tests.txt, because proptest locates regression files by source path.
• src/bin/main.rs tests go to src/bin/main_tests.rs. I added autobins = false so Cargo doesn't try to build that file as a second binary. The explicit [[bin]] steplock target is unchanged.
• CONTRIBUTING.md documents the layout.
• Lint ratchet: clippy::items_after_statements is now deny (no existing violations).

No test logic changed. The only edits are moving code and letting rustfmt re-indent it.

Test plan

• cargo test passes with the same counts as before the move (112 lib, 25 bin, 16 cli, 11 integration)
• cargo clippy --all-targets -- -D warnings passes
• cargo fmt -- --check passes
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps passes
• cargo package --list includes all 12 *_tests.rs files

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