test: synthetic corpora with matcher-independent ground truth

[rikkhill]

What

Adds a synthetic-corpus generator and the suite that consumes it. Test-only — nothing under python/epigrep/ or src/ references it, so invariant 5 holds and the wheel is unchanged.

Why

The existing tests derive their answer key from epigrep itself: apps/epigrep-eval/tasks.json freezes expectations the matcher computed, and the semantic fixtures, while hand-written, are tiny. Both catch drift; neither catches a semantic regression at volume.

crates/epigrep-py/tests/synth.py goes the other way. A PatternSpec describes a pattern; the generator plants episodes that are matches or near-misses by construction and never calls match() or explain(). Each negative violates exactly one clause at exactly one step, which is directly its expected NearMiss reason, so all four reasons are covered without hand-enumeration.

Two noise regimes:

• disjoint — background types the pattern can never mention, so ground truth is exact set equality;
• adversarial — background reusing the pattern's own vocabulary, where ground truth no longer holds and the corpus feeds oracle/compiled parity and metamorphic properties instead.

The answer key has teeth, and that is asserted

Random gaps almost never land on a window's inclusive bound, so the first pass missed a one-unit widening of a three-step window. Planted episodes now sit on the boundaries — first positive at each window's upper bound, window_exceeded negatives missing by exactly 1, predicate values boundary-tight. test_a_single_clause_change_breaks_the_answer_key runs nine one-clause pattern mutations and requires each to break the corpus. A companion test requires adversarial noise to genuinely perturb results, so that regime cannot silently test nothing.

Deliberate v1 restrictions

Each enforced by PatternSpec.validate, with tests asserting the rejection:

• event types must be distinct across a spec (repeated types make a planted event play two roles and break matcher-independent ground truth, so A -> A shapes are not generable);
• predicate literals are integers, so satisfying and violating values are always constructible;
• an unwindowed transition forces one episode per partition, since without a window a start's reach is unbounded.

Equal-timestamp tie-breaking is not covered here; it stays with fixtures 02/03.

Verified locally

Check	Result
pytest crates/epigrep-py/tests/	476 passed, 6 skipped
cargo test	52 + 2 passed
cargo fmt --check, clippy (core + epigrep-py)	clean
check_version_sync.py, diff LICENSE	pass
mkdocs build --strict	pass
apps/ tests	56 passed

🤖 Generated with Claude Code

https://claude.ai/code/session_01CjMbfWJXUgcHTxd4c7BfHM