Remove `mut` and `or_default` from the language

[mjm918]

Description

mut and or_default should be removed entirely from the naml language.

mut

• var is already mutable by default — mut is redundant
• const is immutable — no need for a mut counterpart
• Struct method receivers (pub fn (mut self: T)) should always be mutable — mut adds unnecessary ceremony

Related: #26 (var mut should error), #28 (remove mut from receivers)

or_default

This feature should be removed from the language entirely.

Scope

Areas to clean up across the codebase:

1. Lexer: Remove Keyword::Mut (and Keyword::OrDefault / or_default token if it exists)
2. Parser: Remove mut handling in parse_var_stmt (statements.rs), parse_receiver (items.rs), and any or_default parsing
3. AST: Remove mutable field from Receiver, remove any or_default AST nodes
4. Typechecker: Remove mut/or_default handling, make receivers always mutable
5. Codegen: Remove any mut/or_default codegen paths
6. Docs: Update language-reference.md to remove any mut/or_default references
7. Examples: Remove any usage from example files