Add typed `list[T]` with Python-style list literals

[synapticvoid]

Summary

Introduce a built-in list[T] type with Python-like literals ([]) and a minimal, type-safe method set.

Proposed Syntax

Type syntax

• list[T]

Literal syntax

• Empty list: []
• Non-empty list: [a, b, c]

Method syntax

• xs.len() -> Int
• xs.contains(value: T) -> Bool
• xs.get(index: Int) -> IndexError!T
• xs.add(value: T) -> Unit (mutating)
• xs.remove(index: Int) -> IndexError!T (mutating, remove-at-index)

Type Inference Rules

• const xs = [1, 2, 3] infers list[int]
• Empty list literals require annotation:
  • const xs: list[int] = []
  • var ys: list[str] = []

Type Safety Rules

• All list elements must be assignable to T
• Mixed-type literals are invalid unless explicitly typed as list[any]
• No implicit any inference from list literals

Mutability Rule

• const forbids rebinding
• Mutating methods (add, remove) require a mutable receiver (var)
• Example:
  • Invalid: const xs = [1, 2]; xs.add(3);
  • Valid: var xs = [1, 2]; xs.add(3);

Scope (Minimal)

• Parser support for:
  • list[T] in type positions
  • list literals: [], [x, y]
  • method calls on list values (existing call/field syntax)
• Type checker support for:
  • inference for non-empty list literals
  • element type validation
  • explicit-type requirement for empty list literals
  • list method signatures and mutable-receiver checks
• Interpreter support for:
  • list runtime value
  • method dispatch/intrinsics for len, contains, get, add, remove

Out of Scope

• User-defined generics
• Full list method surface area
• Slices/ranges syntax
• Stack API (push/pop) — reserved for future stack type

Acceptance Criteria

• Valid:
  • const xs: list[int] = [1, 2, 3]
  • var xs = [1, 2, 3]; xs.add(4);
  • const n: Int = [1, 2, 3].len();
• Invalid (type errors):
  • const xs: list[int] = [1, "x"]
  • const xs = [] (no annotation)
  • const xs = [1, 2]; xs.add(3);