The multiplicative comparative frame (PR-5c) admits exactly one shape —
Mul(Symbol, Literal), a unit-bearing symbol times a dimensionless integer
(count × scalar = count). That contract was held by OMISSION: to_relation's
`case _: return None` refused every other Mul shape, but no test would fail
if the guard were loosened, and no doc stated where the guarantee lives.
This makes the obligation meaningfully-failing (CLAUDE.md Schema-Defined
Proof Obligations), with no runtime logic change:
- test_mul_projection_admits_only_symbol_times_literal — Mul(Symbol, Symbol)
(a count×count product), a commuted factor, and compound factors all REFUSE
(to_relation → None). Verified to go red when a Mul(Symbol, Symbol) projection
arm is injected.
- test_literal_factor_is_dimensionless_by_construction — Literal has exactly
one field (value); a unit-bearing literal multiplication is unrepresentable,
not merely unchecked.
- test_scalar_only_guard_is_load_bearing — check_admissibility's `multiply`
dispatch products operand units generally (count×count → count², no refusal),
so it would NOT catch the masquerade. The projection arm is the sole boundary.
Docstrings on Mul and to_relation now state the scalar-only contract and that
it is enforced at the projection boundary, not in the dimensional checker.
Gates unchanged: setup-oracle 15-case 15/0/0 and R1 2/0/8 (setup_wrong=0);
77 expr/admissibility/reader/setup-oracle tests + 56 architectural invariants
green. No serving path touched.
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