core/docs/analysis/r3-2-unit-conversion-2026-06-08.md

R3.2 — explicit time-unit conversion (single-rate)

As of: R3.2, on main + feat/r3-2-conversion.

R3.2 adds exactly one capability to the single-rate organ: a duration whose unit converts to the rate's denominator (minute ↔ hour) is now solved instead of refused. It closes the most common rate gap — 60 miles per hour for 30 minutes → 30 miles — which the R3.1 loop surfaced as a proposal.

Lane state

• R3 gold 13 / 13 valid (7 solved / 2 solver_refuses / 4 reader_refuses)
• R3 reader 9 setup_correct / 0 setup_wrong / 4 refused → answers 7 / 0 / 6
• R1 7/0/3 · R2 10/0/0 · serving unchanged · router-hygiene invariant green · off-serving

Design — Option A (text-faithful model, Fraction in the solver)

• Model (RateProblem) gains time_unit — the duration's original unit from the text; time stays the original int. rate_unit.denominator is the target unit for composition. The setup remembers what the text said ("30 minutes"), not a normalized internal.
• Conversion (conversion.py): convert_time(value, from, to) -> Fraction — exact rational (fractions.Fraction), identity for the same unit (incl. non-time like box), minute ↔ hour otherwise, refuse else. No floats.
• Solver confines Fraction: rate × convert(time) (or the inverses), then exact-int-or-refuse (non_integer_solution). A non-convertible duration raises ConversionError → rate_unit_mismatch.
• Reader accepts a convertible mismatch (keeps the original time_unit; the solver converts); a non-convertible mismatch still refuses rate_unit_mismatch.
• Signature includes time_unit — "30 minutes" and "30 hours" are different setups at the same rate.

Gold change

• r3-09 flips reader_refuses → solved: "60 mph for 30 minutes" → 30 minute = Fraction(1,2) hour → 60 × 1/2 = 30 mile (the never-convert distractor 1800 is rejected).
• New r3-13 (non-convertible): "…per hour for 3 gallons" → rate_unit_mismatch (gallon is not a time unit). Stays a proposal surface.

Failure-family / hygiene (confirmed, no code change)

• rate_unit_mismatch now fires only for non-convertible durations → still maps to the unsupported_rate_duration growth surface (propose adding that unit pair). Convertible mismatches now solve (no longer a failure). temporal_state / not_rate_shaped unchanged.
• The router-organ-hygiene invariant stays green — R3 still refuses foreign R1/R2 text as input_shape, never a substantive boundary.

Acceptance (met)

R3 setup_correct 8 → 9 · answer_wrong = 0 · r3-09 refused/proposal → solved · non-convertible mismatch still refused/proposal · R1/R2 unchanged · no float path (Fraction → int-or-refuse).

Deferred (named for later)

length (mile ↔ km), currency (dollar ↔ cent), compound conversions, combined/multi-rate, clock-time intervals. The dimensional substrate is now proven for exact rational time conversion.