Previously, `elabInitialize` only checked for explicit `private` when
deciding whether to mangle `fullId`, ignoring the `module` system's
default-private semantics. It also overrode the user's visibility for
the generated `initFn` via `visibility.ofBool`.
Now, `elabInitialize` uses `elabVisibility` + `isInferredPublic` to
correctly handle all visibility contexts. The generated `initFn`
inherits the user's visibility rather than being forced public.
Also factors out `elabVisibility` from `elabModifiers` for reuse.
