feat: add hd and tl builtins

[danieljohnmorris]

Summary

• Add hd builtin: returns the first element of a list or first character of text
• Add tl builtin: returns a list without its first element, or text without its first character
• Both error on empty input (ILO-R009)

Changes

• Verifier: hd and tl in BUILTINS array with list_or_text param type; builtin_check_args returns correct types (e.g. hd on L n returns n, tl on L n returns L n)
• Interpreter: runtime dispatch for both builtins on List and Text values
• VM: OP_HD (48) and OP_TL (49) opcodes with compiler emission and NanVal dispatch

Test plan

• Verifier: hd/tl valid with list, valid with text, wrong type errors
• Interpreter: hd/tl on list and text
• VM: hd/tl on list and text
• All 865 tests pass (753 unit + 112 integration)
• Clippy clean (no new warnings)

[danieljohnmorris]

Superseded by #56 which combines all builtins + ternary with correct opcode numbering.