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>

1 files changed, 12 insertions, 7 deletions

diff --git a/ui/windows/channels_window.py b/ui/windows/channels_window.py
index 266432b..597b0b2 100644
--- a/ui/windows/channels_window.py
+++ b/ui/windows/channels_window.py
@@ -618,8 +618,8 @@ _KIND_HELP = {
         "Example:\n    offset = vars.get('zero', 0)\n    return x[0] - offset"
     ),
     "custom_script": (
-        "Full Python.  Must define:  def compute(x, t): ...\n"
-        "Available globals: x, signal names, t, math, vars, state"
+        "Multi-line Python block.  Assign computed value to result.\n"
+        "Available: x, signal names, t, math, vars, state"
     ),
 }
 
@@ -758,9 +758,11 @@ class DerivedBlock(QFrame):
             from ui.code_templates import BUILTIN_TEMPLATES
             txt = BUILTIN_TEMPLATES.get(self.dc.kind, "")
         elif self.dc.kind == "expression":
-            txt = self.dc.expression
+            from ui.code_templates import SCRIPT_TEMPLATES
+            txt = self.dc.expression or SCRIPT_TEMPLATES.get("expression", "")
         else:
-            txt = self.dc.script
+            from ui.code_templates import SCRIPT_TEMPLATES
+            txt = self.dc.script or SCRIPT_TEMPLATES.get(self.dc.kind, "")
         self._code.setPlainText(txt)
         code_lay.addWidget(self._code)
         # "Edit as Custom Script" button — only visible for built-in kinds
@@ -875,16 +877,19 @@ class DerivedBlock(QFrame):
         # expression kind: label-only, no code injection (single-line expr)
 
     def _on_kind(self, idx: int):
-        from ui.code_templates import BUILTIN_TEMPLATES
+        from ui.code_templates import BUILTIN_TEMPLATES, SCRIPT_TEMPLATES
         k = _ALL_KINDS[idx]
         self.dc.kind = k
         self._help.setText(_KIND_HELP.get(k, ""))
         if k in ("derivative", "second_derivative") and not self._src_combos:
             self._add_src()
         if k in _BUILTIN:
-            # Reset to bare template first; _update_vars_hint will prepend comment
+            # Reset to bare template; _update_vars_hint will prepend variable comment
             self._code.setPlainText(BUILTIN_TEMPLATES.get(k, ""))
-        self._update_vars_hint()  # also injects variable comment into code
+        elif k in _SCRIPT:
+            # Show starter template for script kinds (always replace on kind switch)
+            self._code.setPlainText(SCRIPT_TEMPLATES.get(k, ""))
+        self._update_vars_hint()
         self._update_visibility()
 
     def _on_wrt_changed(self, idx: int):