Fix custom_script execution + add templates for expression/function kinds

- custom_script now uses exec-style (assign to `result`) matching the
  built-in templates — previously auto-wrapped in def compute() which
  had no return statement, causing state and channel vars to silently fail
- function kind unchanged: auto-wraps body as def compute(x,t); use return
- Add SCRIPT_TEMPLATES for expression, function, custom_script kinds;
  shown automatically when user switches to a script kind (replacing stale
  code from previous kind)
- New channels default to template when no saved code exists

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions