test: data-driven filter test engine with RTK_UPDATE_TEST_DATA

[osipxd]

Summary

• Adds src/datatest.rs: a lightweight data-driven test engine that discovers input/expected file pairs under tests/data/ and runs them through a caller-supplied closure returning the actual output
• Adds Rust and Python test data under tests/data/filter/ covering the full MinimalFilter and AggressiveFilter codepaths, and wires filter::tests::test_filter to use it

Update mode: set RTK_UPDATE_TEST_DATA=1 to overwrite expected files with the actual filter output instead of failing — useful when filter behaviour intentionally changes.

Known filter quirks documented by the test data (noted for follow-up):

• pub(crate) items are silently dropped by aggressive (pub\s+ regex doesn't match pub(crate)), but Python __internal_method is kept — asymmetry between the two languages
• Closing } is dropped from struct/mod bodies (brace depth tracking exits before emitting it)
• #[cfg(test)] mod tests wrapper is dropped but its contents (use super::*, function signatures) are still emitted at the outer indentation level
• Test methods inside a unittest.TestCase subclass are not dropped in Python either
• // ... implementation placeholder is used for Python files (C-style comment)
• Implementation placeholder is always indented 4 spaces regardless of nesting depth

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• RTK_UPDATE_TEST_DATA=1 cargo test filter::tests::test_filter — verified update mode writes correct expected files, re-run confirms green

[CLAassistant]

All committers have signed the CLA.

[osipxd]

The failure report currently dumps full expected/actual blocks. A line-by-line diff would be much easier to read, but that requires adding a diff crate (e.g. similar). Leaving as-is for now to keep dependencies minimal.