perf: add periodicity checking

CLAUDE.md

@@ -113,6 +113,13 @@ pixel is a handful of `f32` complex multiplies.
   use, and `tests/shader_valid.rs` compiles every kind × Julia × DE variant to
   SPIR-V. So a new kind needs no pipeline-list change, only its `KIND_*`
   constant. `buddhabrot.wgsl` does the same with its own `override KIND`.
+  Interior pixels exit early through **periodicity detection**. It uses
+  Brent-style checkpoints plus two guards: the cycle's multiplier must be
+  clearly attracting (`PERIOD_MAX_MULT2`), and the contracting return must
+  repeat in `PERIOD_CONFIRMATIONS` consecutive windows. Both guards are
+  needed: without them, exterior pixels at cusps and minibrot edges turned
+  black. Retune them only against f64 ground truth on such views. Phoenix is
+  excluded (two-term map).
   `advance_delta(z, e)` is the per-kind delta step (`z` = reference point,
   `e` = current delta); the caller adds `step_add` (= `dc`) afterward — this
   relies on `c` being additive in every current kind's formula (a kind where