Add explicit swift-nio dependency for Xcode 26 transitive dylib bug

[stovak]

Problem

Xcode 26 introduced a regression in Swift Package Manager builds where packages built as dynamic frameworks don't properly propagate transitive framework dependencies during the link phase.

Symptoms

When EventSource is built with the AsyncHTTPClient trait enabled, the build fails with:

Undefined symbols for architecture arm64:
  NIOCore symbols referenced from EventSource.o

Root Cause

• Xcode 26 builds all "automatic" SPM products as dynamic frameworks (*.framework)
• EventSource (with AsyncHTTPClient trait) uses symbols from NIOCore (via inlined code from AsyncHTTPClient)
• However, EventSource's Package.swift doesn't declare swift-nio as a direct dependency
• It only declares async-http-client, which itself depends on swift-nio
• Xcode 26 correctly links NIOCore.framework into AsyncHTTPClient.framework
• BUT Xcode 26 fails to propagate this transitive dependency when linking EventSource.framework
• Result: EventSource.o references NIOCore symbols but the framework doesn't link against it

Solution

Add explicit package-level and target-level dependencies for swift-nio to ensure Xcode 26 can link the framework correctly.

Changes

• Added .package(url: "https://github.com/apple/swift-nio.git", from: "2.81.0") to package dependencies
• Added .product(name: "NIOCore", package: "swift-nio") to EventSource target dependencies

Impact

• No functional changes - NIOCore was already available transitively through AsyncHTTPClient
• Fixes builds for projects using EventSource with AsyncHTTPClient trait under Xcode 26
• No impact on Xcode 25 or earlier (explicit dependency is redundant but harmless)
• No impact on Linux/Swift CLI builds (they don't have the transitive dylib bug)
• No impact on EventSource builds without the AsyncHTTPClient trait (dependency is only used when trait is enabled)

Testing

Verified that this fixes builds for projects using EventSource with the AsyncHTTPClient trait as a transitive dependency under Xcode 26.

Previously failed with undefined symbols, now builds and links correctly.

Notes

• Used swift-nio 2.81.0 as minimum version to match async-http-client's requirement
• This is defensive programming - making implicit transitive dependencies explicit

---

Related Issue: This is a workaround for an Xcode 26 regression. Ideally Apple would fix the SPM dylib linking behavior, but adding explicit dependencies is a reasonable defensive practice regardless.

[mattt]

Gosh, this again? Looks like I'd do well to go through every project I have using async-http-client to test that Xcode actually works.

But seriously, thank you for reporting huggingface/swift-xet#6 and opening this PR. I really appreciate it!

[mattt]

@stovak Looks like this is failing the lint check. I tried applying a fix to your PR branch but got a 403. Can you please run swift format -i -r . and push that change?

[stovak]

@mattt done.

Thank my intern, Claude. 😉

[mattt]

Thanks again for your help, @stovak! This is now available in 1.4.1.