v0.1.18

docs(CLAUDE): document the parity harness + how to read the %differ

Explains running scripts/parity, the one-process JVM / per-screen native
mechanics, the build/parity outputs, and — the load-bearing part — what
the number means: it's a font-drift-inflated ranking signal, not a
pass/fail score. Ghosted text + dark shapes = normal; a solid bright
block or one-sided shape = regression. Baseline gradient ~2%..32%; known
SDL gaps (solid gradients, effects) inflate specific screens.