Adds crates/headroom-core/src/transforms/kompress.rs (672 lines): the Kompress
ML prose compressor ported to Rust, running the ModernBERT tokenizer plus the
kompress-v2-base ONNX model through ort with a cache-only loader that never
touches the network.
Parity-only. Nothing calls it: the only references outside the module are the
pub mod / pub use declarations in transforms/mod.rs. live_zone.rs still carries
TODO(PR-B4), so PlainText dispatch remains a no-op and Python continues to serve
prose compression. The pyo3 bridge is untouched and no Python source changes, so
the new engine is unreachable from the shipped package. #1155 wires it up.
Ships 21 recorded parity fixtures, a KompressComparator in headroom-parity, and
scripts/record_kompress_fixtures.py.
Verified byte-identical to the recorded Python output:
[kompress] total=21 matched=21 skipped=0 diffed=0
That required ONNX Runtime >= 1.24 (see #2591) — below it ort deadlocks instead
of erroring, which is why these fixtures had never been run. In CI the model is
absent from the HF cache, so the comparator errors and the fixtures report
Skipped rather than hanging.
Also gates the module behind the ml feature, matching magika_detector: kompress.rs
uses ort, which is optional = true, so an unconditional pub mod broke
cargo check --no-default-features (the static-musl path). CI does not catch that
class of break because cargo test --workspace only builds default features.
