Add method-call sugar for collection builtins (`xs.len()` style)

[synapticvoid]

Task

Implement method-call sugar so that xs.len() desugars to len(xs), etc. Applies to all three collection types: List[T], Map[K, V], and Set[T].

Desugaring table

Method form	Desugars to
xs.len()	len(xs)
xs.contains(v)	contains(xs, v)
xs.get(i)	get(xs, i)
xs.get(i, d)	get(xs, i, d)
xs.add(v)	add(xs, v)
xs.remove(i)	remove(xs, i)

Sugar only — behavior and types match the builtin forms exactly.

Acceptance criteria

• Parser recognizes expr.method(args) on collection types
• AST desugars to builtin call before type-checking
• Works for List[T], Map[K, V], and Set[T]

Spec

See PDP-009: Collection Types.