materialized-files

The byte-equality writer for derived harness files — a correct target is left untouched, while a differing target is replaced exactly once.

Derived harness artifacts are a target state, not an instruction to write. The one writer compares the existing bytes with the desired bytes and writes only on a difference; copied package files use the same rule. This gives all materialize consumers one operational idempotence law: a correct target keeps its filesystem identity, while a changed target receives exactly the requested bytes. It does not decide which paths belong to a materialize pass or which stale paths to remove; those are the owning renderer's target map.