feat: add cat builtin — join list of text with separator

[danieljohnmorris]

Summary

• Adds cat builtin: joins a list of text with a separator string, e.g. cat ["a","b","c"] "," returns "a,b,c"
• Signature: cat(L t, t) -> t
• Implemented across verifier (type checking), tree-walk interpreter, and register VM (OP_CAT opcode 48)

Test plan

• Verifier: valid call, wrong type arg1 (number instead of list), wrong arity
• Interpreter: basic join, empty list
• VM: basic join, empty list
• All 858 tests pass (746 unit + 112 integration)
• cargo clippy clean (no new warnings)

[danieljohnmorris]

Superseded by #56 which combines all builtins + ternary with correct opcode numbering.