fix(transpiler): @when result assignment silently dropped in exported module

[xemishra]

Summary

WhenTransformer.visit_FunctionDef in transpiler.py was returning
ast.Expr(ast.Assign(...)) an ast.Assign statement incorrectly
wrapped inside an ast.Expr node.

This caused a silent double failure:

• ast.Assign is a statement, not an expression, so the wrapping
  violates the Python AST contract.
• visit_Module filters out every ast.Expr node to strip bare
  expression statements. Because the return value matched that filter,
  the name = whencall(...) assignment was thrown away entirely.

Impact

For every @when-decorated function, the variable that should hold
the result Cown was never assigned in the exported module. Any code
that read .value, checked .exception, or chained behaviors on the
result was operating on None (or whatever the name happened to be
bound to previously), with no error at schedule time.

Root cause

# Before (buggy)
return ast.Expr(ast.Assign([ast.Name(id=node.name)], when_call))

Fix

# After
assign = ast.Assign(
    targets=[ast.Name(id=node.name, ctx=ast.Store())],
    value=when_call,
)
ast.copy_location(assign, node)
ast.fix_missing_locations(assign)
return assign

Returning a bare ast.Assign passes the visit_Module filter
correctly, satisfies the AST node contract, and ensures the result
Cown is bound to the user's chosen name in the exported module.

Testing

Verified by invoking export_module() directly on a minimal source
containing a @when-decorated function and asserting that
name = whencall(...) appears in the unparsed output, which it did
not before this fix.