Implement tuples with (i32, string) syntax

[nbeerbower]

Add first-class tuple support across interpreter and compiler:

• AST: Add TYPE_TUPLE and EXPR_TUPLE_LITERAL with element_types array
• Parser: Parse tuple type syntax (type1, type2) and literal syntax (val1, val2)
• Parser: Handle .0, .1 numeric property access for tuple indexing
• Interpreter: Add VAL_TUPLE, Tuple struct, and tuple operations
• Interpreter: Support tuple type coercion for function params/returns
• Compiler: Add CHECKED_TUPLE, code generation for tuple literals and access
• Runtime: Add HML_VAL_TUPLE, HmlTuple struct, and hml_tuple_get
• Bundler: Handle EXPR_TUPLE_LITERAL in dependency collection
• Parity test: tests/parity/language/tuples.hml covering all tuple features

Usage:
let pair = (42, "answer");
let typed: (i32, string) = (100, "hello");
print(pair.0); // 42
print(pair[1]); // answer
print(pair.length); // 2