🧪 Cover NewEmpty's documented empty-interpreter contract

[ccamel]

Summary

NewEmpty documents an interpreter with no predicates or operators in interpreter.go:23-28, but root tests do not call it and coverage.txt:2 records its constructor branch as uncovered.

Why it matters

The public empty-interpreter constructor needs an executable contract that distinguishes it from New and prevents accidental bootstrap or builtin registration.

Scope

Add TestNewEmpty to interpreter_test.go for the documented empty state.

Acceptance criteria

• NewEmpty().QuerySolution("missing.").Err() renders error(existence_error(procedure,missing/0),root).
• The test demonstrates that NewEmpty has not registered builtins.
• The test demonstrates that NewEmpty has not loaded bootstrap predicates or operators.

Verification

Run go test . -run '^TestNewEmpty$' and assert the exact rendered error from the missing zero-arity procedure query.