Fix factorial forward references by deferring UInt view return types

[jmh530]

A standalone factorial(3) call can fail with an Fp!128 forward-reference error, including when reached through mir-stat's integer-shape gamma PDF. Inferring the return types of both UInt.view overloads defers the BigUIntView instantiation that exposes this cycle. The method bodies, concrete view types, qualifiers, and attributes remain unchanged.

Add mutable/const view tests and a separate factorial regression executable. The standalone caller is important: prior bignum instantiations in the normal library test suite can mask the failure. Run it in the existing Windows, Linux, and macOS CI jobs.

Validation:

• Confirmed the standalone DUB regression fails before the fix and passes afterward with Windows DMD 2.113 and LDC 1.43.
• LDC bignum test configuration: 12 modules passed.
• DMD general/numeric test configuration: 51 modules passed.
• The same return-type change was also checked in isolated Linux DMD 2.112 and LDC 1.36 reproductions during investigation; hosted CI will exercise the submitted workflow.

Fixes #485.

AI-Assisted-By: Codex (GPT-6 Astra Medium)