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

[synapticvoid]

Summary

Support method-call syntax as sugar over existing collection builtins.

Proposal

• xs.len() desugars to len(xs)
• xs.contains(v) desugars to contains(xs, v)
• xs.get(i) desugars to get(xs, i)
• xs.add(v) desugars to add(xs, v)
• xs.remove(i) desugars to remove(xs, i)

Rules

• Sugar only: behavior and types must match the builtin forms exactly
• Mutability checks are unchanged (e.g. add/remove require mutable receiver)
• No user-defined extension methods in this issue

Scope (Minimal)

• Parser: keep existing obj.method(args) shape
• Type checker: resolve supported collection methods to builtin signatures
• Interpreter: dispatch method calls to builtin implementations

Acceptance Criteria

• const n = [1,2,3].len(); behaves like len([1,2,3]);
• var xs = [1,2]; xs.add(3); behaves like add(xs, 3);
• const xs = [1,2]; xs.add(3); fails with mutability error
• Unknown methods (e.g. xs.foo()) fail with type error