* feat(generator): warn on low-diversity corpora

Surface the silent-degenerate failure mode that rstr's non-greedy bug
hit on kubernetes_secret_yaml pre-0.2.8 (27 samples / 10 unique middles,
all sharing one degenerate core).

Parent-side pass after corpus collection: per-rule middle-50-char slice
uniqueness, warn when ratio < 0.4 and samples >= 10. Works under both
sequential and parallel backends (spawn workers don't forward logs).
Threshold lets inherently narrow patterns (inst_tag at 0.53) through
without noise.

* ci: matrix lint across supported Python versions

Local dev on 3.14 and CI lint on 3.12 drifted: mypy passes on one,
flags on the other (method-assign + warn_unused_ignores interaction).
Expand the matrix so CI surfaces both sides of the drift.

Also adds 3.14 to the test matrix and classifiers — local already
runs it clean, just hadn't been declared supported.

* Release 0.2.9: diversity warning + CI lint matrix