Remove `mut` requirement from struct method receivers

[mjm918]

Description

Currently, struct methods that mutate self require mut in the receiver:

pub fn (mut self: Counter) increment() {
    self.count = self.count + 1;
}

Without mut, mutation of self fields fails. This adds unnecessary ceremony — self in methods should always be mutable.

Proposal

Make self always mutable in method receivers. Remove the mut keyword from receiver syntax entirely.

Before: pub fn (mut self: Counter) increment()
After: pub fn (self: Counter) increment()

Affected Code

• Parser: namlc/src/parser/items.rs — parse_receiver() checks for Keyword::Mut and sets Receiver.mutable
• Type checker: namlc/src/typechecker/mod.rs — uses recv.mutable when defining self in scope via env.define()
• AST: Receiver struct has a mutable: bool field

Fix

1. In parse_receiver(): remove mut keyword check, always set mutable: true (or remove the field)
2. In typechecker: always define receiver as mutable
3. Optionally emit a warning/error if mut is still used, for a deprecation period