[ScottishTapWater]

Summary

Stacked on #28 (feat/nullable-discovery).

• Recognise nullable numeric scalar numerators when discovering scalar/quantity division.
• Reuse the existing reciprocal operator and C# nullable lifting.
• Preserve the existing implicit numeric conversion restrictions, including rejection of nullable decimal.
• Add discovery and execution regressions covering nullable/non-nullable divisors and null propagation.

Verification

• Before the fix: 12 new regression cases failed; the nullable decimal rejection case passed.
• After the fix: all 13 new cases passed; both affected test classes passed (144/144).
• Build: zero warnings/errors.
• Formatting verification and git diff --check passed.
• Independent C# review: no findings.