### Motivation:

NIOIMAPCore's parser guards the libc import per file with
Darwin/Glibc/Musl/Windows and otherwise falls into
`#else let badOS = { fatalError("unsupported OS") }()`. On the Swift
Android SDK none of those arms match, so every guarded file takes the
`#else` branch and the module fails to build with "invalid redeclaration
of 'badOS'" (the symbol is declared at file scope in ~17 sources).

### Modifications:

Add an `#elseif canImport(Android)` arm importing `Android` to the per-file
libc-import guards in NIOIMAPCore. For `UInt8+ParseTypeMembership.swift`,
which imports specific ctype functions, add `import func Android.isalnum` /
`import func Android.isalpha`.

### Result:

NIOIMAPCore compiles for Android. No change on any other platform.

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