Mockolo parses an implicitly-unwrapped optional (`Int!`) as `Optional<Int>` with
an `isIUO` flag, and `SwiftType.description` rendered its single optionality twice
— the optional-sugar case appended `?` and the `isIUO` block appended `!` —
producing `Int?!` (≈ `Int??`), which does not satisfy the `Int!` (≡ `Int?`)
protocol requirement, so the generated mock failed to conform. This was
pre-existing; no fixture exercised an IUO declaration (compiled `@Fixture` mocks
surfaced it).

- Properties: suppress the duplicate `?` for IUO in the optional-sugar case; the
  `isIUO` block still appends `!`, so the type renders `Int!`. Inert for everything
  else — plain `Int?` is unchanged, and the intentional `SomeProtocol!` force-unwrap
  backings go through the default nominal branch.
- Functions / subscripts / generic returns: `!` is illegal inside a closure type, so
  the synthesized handler closure (`((Int!) -> Int!)?`) was non-compiling. Clear
  `isIUO` on the handler's argument and return types in `toClosureType` so they
  render as plain optionals (`((Int?) -> Int?)?`, legal), while the declared
  signature keeps `Int!` and still conforms. Clearing `isIUO` is a no-op for non-IUO
  types, so no existing handler changes.

Adds compiled fixtures proving conformance: `iuoVars` (get-only + get-set) in
TestVars and `iuoFuncs` (func param/return + subscript) in TestFuncs. No existing
fixture output changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>