build: Elide NIOPosix for WASI platforms only (#3485)

### Motivation:

Packages that consume NIOPosix should be able to compile to WASI
platforms without special configurations. This change elides the
NIOPosix source from WASI platforms to simplify configuration.

Without this change:

```swift
dependencies: [
    // Without this PR, downstream packages must maintain exhaustive platform list to exclude `.wasi`:
    .product(
        name: "NIOPosix",
        package: "swift-nio",
        condition: .when(platforms: [
            .macOS,
            .macCatalyst,
            .iOS,
            .tvOS,
            .watchOS,
            .visionOS,
            .driverKit,
            .linux,
            .windows,
            .android,
            .openbsd,
            // .wasi // <-- Need to exclude this, because there is no exclusion list api for SPM conditionals
        ])
    ),
]
```

With this change:

```swift
dependencies: [
    // Without this PR, downstream packages can consume NIOPosix simply:
    .product(name: "NIOPosix", package: "swift-nio"),
]
```

### Modifications:

- Add compiler directives (`#if !os(WASI)`) to all source files in
NIOPosix

### Result:

Downstream packages can compile to wasm without manually excluding the
NIOPosix dependency.

### Testing performed

- Verified `swift build --swift-sdk wasm32-unknown-wasip1 --target
NIOPosix` compiles, which demonstrates proper elision of source files in
NIOPosix that aren't wasm-ready.
- Confirmed GitHub [checks
pass](https://github.com/PassiveLogic/swift-nio/actions/runs/21151341738).

Author: scottmarchant

Sources/NIOPosix/BSDSocketAPICommon.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import NIOCore
 
 #if os(Windows)
@@ -391,3 +394,4 @@ enum NIOBSDSocketControlMessage: _BSDSocketControlMessageProtocol {}
 
 /// The requested UDS path exists and has wrong type (not a socket).
 public struct UnixDomainSocketPathWrongType: Error {}
+#endif  // !os(WASI)

Sources/NIOPosix/BSDSocketAPIPosix.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import NIOCore
 
 #if os(Linux) || os(Android) || os(FreeBSD) || canImport(Darwin) || os(OpenBSD)
@@ -426,3 +429,4 @@ extension msghdr {
     }
 }
 #endif
+#endif  // !os(WASI)

Sources/NIOPosix/BSDSocketAPIWindows.swift

@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if !os(WASI)
+
 import NIOCore
 
 #if os(Windows)
@@ -803,3 +805,4 @@ extension WSAMSG {
     }
 }
 #endif
+#endif  // !os(WASI)

Sources/NIOPosix/BaseSocket.swift

@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if !os(WASI)
+
 import CNIOLinux
 import CNIOOpenBSD
 import NIOConcurrencyHelpers
@@ -420,3 +422,4 @@ func __testOnly_withMutableSockAddr<ReturnType>(
 ) rethrows -> ReturnType {
     try addr.withMutableSockAddr(body)
 }
+#endif  // !os(WASI)

Sources/NIOPosix/BaseSocketChannel+SocketOptionProvider.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import NIOCore
 
 extension BaseSocketChannel: SocketOptionProvider {
@@ -83,3 +86,4 @@ extension BaseSocketChannel: SocketOptionProvider {
         try self.socket.getOption(level: level, name: name)
     }
 }
+#endif  // !os(WASI)

Sources/NIOPosix/BaseSocketChannel.swift

@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if !os(WASI)
+
 import Atomics
 import NIOConcurrencyHelpers
 import NIOCore
@@ -1435,3 +1437,4 @@ func executeAndComplete<Value: Sendable>(_ promise: EventLoopPromise<Value>?, _
         promise?.fail(e)
     }
 }
+#endif  // !os(WASI)

Sources/NIOPosix/BaseStreamSocketChannel.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import NIOCore
 
 class BaseStreamSocketChannel<Socket: SocketProtocol>: BaseSocketChannel<Socket>, @unchecked Sendable {
@@ -301,3 +304,4 @@ class BaseStreamSocketChannel<Socket: SocketProtocol>: BaseSocketChannel<Socket>
         }
     }
 }
+#endif  // !os(WASI)

Sources/NIOPosix/Bootstrap.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import CNIOLinux
 import CNIOOpenBSD
 import NIOCore
@@ -2701,3 +2704,4 @@ private struct DefaultNIOPipeBootstrapHooks: NIOPipeBootstrapHooks {
         try PipeChannel(eventLoop: eventLoop, input: input, output: output)
     }
 }
+#endif  // !os(WASI)

Sources/NIOPosix/ControlMessage.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import NIOCore
 
 #if canImport(Darwin)
@@ -438,3 +441,4 @@ extension AddressedEnvelope.Metadata {
         )
     }
 }
+#endif  // !os(WASI)

Sources/NIOPosix/DatagramVectorReadManager.swift

@@ -11,6 +11,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if !os(WASI)
+
 import CNIODarwin
 import CNIOLinux
 import CNIOOpenBSD
@@ -296,3 +299,4 @@ extension UnsafeMutableBufferPointer {
         rawPointer.deallocate()
     }
 }
+#endif  // !os(WASI)