The existing tests for self keyword and argument-position builtin calls only asserted absence of errors on valid input, leaving room for the positive cases to pass silently even if type propagation was broken.

Changes

• Negative assertions in test_self_keyword_preserves_piped_type: three new !is_empty() checks confirm the type checker correctly rejects incompatible pipeline chains:

"hello" | self | sort   # string → self → array-only builtin: must error
42      | self | sort   # number → self → array-only builtin: must error
"hello" | upcase | sort # confirms propagation through the whole chain, not just self

These cases would silently pass if self discarded the piped type or if overload resolution was simply deferred without actual unification.

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