feat: add has builtin — membership test for lists and text

[danieljohnmorris]

Summary

• Adds has builtin that checks if a collection contains a value
• Works with lists (element membership via equality) and text (substring search)
• Implemented across all three execution paths: verifier, tree-walking interpreter, and VM
• New opcode OP_HAS (48) for VM with ABC encoding: R[A] = has(R[B], R[C])

Test plan

• Verifier: valid list arg, valid text arg, wrong type arg 1 (3 tests)
• Interpreter: list contains, list not contains, text contains, text not contains (4 tests)
• VM: list contains, list not contains, text contains, compiles to OP_HAS (4 tests)
• All 862 tests pass
• Clippy clean (no new warnings)

[danieljohnmorris]

Superseded by #56 which combines all builtins + ternary with correct opcode numbering.