Root cause: default step-idx was 9, but the expression has 16 steps.
At step 9, only "the joy" + empty emerald span renders. Changed default
to 16 so all four words display after hydration.
Reverted mutable-list changes — (list) already creates ListRef in the
OCaml kernel, so append! works correctly with plain (list).
Added spec/tests/test-stepper.sx (7 tests) proving the split-tag +
steps-to-preview pipeline works correctly at each step boundary.
Updated Playwright stepper.spec.js with four tests:
- no raw SX visible after hydration
- default view shows all four words
- all spans inside h1
- stepping forward renders styled text
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
