Strengthen QEC sizing and distance contracts with Flux

[arniber21]

What to build

Make QEC sizing helpers express their actual legal domains and successful formulas. Repetition graph construction, surface/repetition atom counts, and ceiling division must not rely on unchecked subtraction or result types with no refined postcondition.

Acceptance criteria

• Repetition graph construction requires a valid nonzero/minimum distance or returns QldpcError without underflow.
• Surface-code sizing proves the successful result equals 2*d*d - 1 for valid odd d >= 3 without overflow.
• Repetition-code sizing proves the successful result equals 2*d - 1 for valid d >= 2 without overflow.
• Ceiling division proves its successful result is the mathematical ceiling for nonzero denominators.
• Invalid and overflow inputs return the documented typed errors in stable builds.
• Flux checks these postconditions without empty Result<_, _> specifications or new trusted bodies.

Blocked by

• Restore complete and deterministic Flux CI coverage #404

[arniber21]

Implemented and merged via PR #460. Closing as complete.