feat: add lexical-mutation corpus generator

[3leapsdave]

Adds a lexical lane alongside the existing crypto fixture lanes: a deterministic generator for synthetic term-matching benchmark corpora.

Grammar

Every term is an immutable zzlx anchor followed by a seed-derived base32 body. Transforms only ever touch body scalars, so a mutated variant is still greppable as synthetic. Two shapes are deliberate exceptions, both documented: common-word decoys, where an ordinary word is the whole point of the control, and below-policy-length terms, which are too short to hold a full anchor.

The matrix

Seven surfaces with per-surface transform classes and severity mixes — 49 required cells, six of which carry a pinned severity rather than the surface rotation. Unicode classes, vowel-drop, and truncation outside snake_case are implemented but sit outside the required matrix behind --include-extensions.

Population floors are enforced inside generation: a run that cannot meet them fails rather than emitting a thin corpus. A default run produces 721 cases — 588 positives, 49 negative controls, and 84 below-policy-length cases.

Output planes

Generation writes a sterile plane (opaque identifiers, answer-key coordinates, digests) and a protected plane (term values and the artifacts a detector scans). The fixture set references the protected manifest by SHA-256 alone. docs/lexical-matrix.md states plainly that this is a handling boundary rather than a confidentiality control: generation is deterministic from a seed the sterile plane carries, so the protected plane is reconstructible by anyone with this source.

Correctness guards

• A positive case whose rendered variant is byte-identical to the unmutated term is rejected at generation time. Guarding the rendered form rather than the token slice is deliberate — a transform can change tokens and still render identically, which is how that class of defect hides.
• Term values are checked for collisions against each other and against a common-word list, covering rendered variants as well as base terms.
• Any 32-bit seed produces a corpus; transform applicability is established after token splitting, since splitting decides which scalars end up adjacent.

Ownership markers

Each generator now owns a distinct output-root marker, so --force can only replace a root its own lane created and a corpus holding no key material is no longer labelled with the marker that locates key-bearing roots. The marker kind is the authorization attribute; the tool field is provenance only. A corpus is built alongside its target and swapped in once complete, so an interrupted run leaves the previous one intact.

Verification

make check-all less the decernor contract lane, which needs a binary at the pinned commit on the running host. Package coverage 87.0%, CLI 80.9%. Determinism verified across separate processes: identical seed produces byte-identical output trees.

🤖 Generated with Claude Code