Fix array callback compatibility for function declaration refs

[memburg]

What changed

• Extended callback argument normalization to function declaration references used in array callback methods
• Added callback-arity metadata plumbing for function references resolved via the interpreter
• Kept strict arity for regular non-callback function calls
• Added regression specs for map(double), reduce(sum, 0), and undefined padding behavior
• Updated reference docs for callback argument behavior

Validation

• Ran crystal spec

Closes #10
Closes #8