Introduce importability as a Swift package (C library). #790

This allows macOS IME developers to simply import LibChewing as a Swift Package.

The added Swift Unit Test files are only for making sure that C APIs are accessible from Swift.

On macOS, as long as libchewing is compilable using cargo, one can always run swift tests to build and run Swift package on macOS with Xcode 26 installed (or standalone Swift 6.1+ installation). Earlier Xcode is okay as long as its shipped Swift is 6.1+.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.06%. Comparing base (c04ae27) to head (3a3df8b).
⚠️ Report is 1 commits behind head on master.

@@ Coverage Diff @@
## master #790 +/- ##
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 Coverage 92.06% 92.06% 
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 Files 37 37 
 Lines 9253 9253 
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 Hits 8519 8519 
 Misses 734 734 

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Thanks for the contribution!

I'm not familiar with Swift. Is it possible to put everything related to Swift under the swift/ top level directory?

It would be good to run the tests in the CI.

@kanru I'm afraid that the Package.swift has to be situated at the root folder of the repo. You can fact-check this with Claude Opus, Codex, Raptor... or whatever AI coding agent you trust.

Other files are relocateable.

It would be good to run the tests in the CI.

I'll give it a try.

@kanru

• All files are relocated to {REPO_ROOT}/swift folder, excepting the mandatory Package.swift.
• CI tests against this Swift Package are added. For now, these tests only run on macOS platform (macOS 26 image). If future modifications are needed, maintainers may delegate the modification tasks to GitHub copilot instead in case no one is available for this task.
• All CI tests are green now: http://github.com/chewing/libchewing/actions/runs/20565593988/job/59063310287?pr=790 despite a useless warning "This header is deprecated. Include <chewing.h> directly.".

@kanru

1. {REPO_ROOT}/tools/CargoBuildPlugin/Plugin.swift should be OK to be be removed. This is something I forgot to remove.
2. In {REPO_ROOT}/swift/tools/CargoBuildPlugin/Plugin.swift we can simplify the following error message:

"`cargo` not found on the system. Please install Rust (https://rustup.rs/) to enable automatic builds, or disable automatic Cargo build by setting `LIBCHEWING_AUTO_BUILD_CARGO=0` and run `swift/scripts/build-cargo.sh` manually to produce the library before running `swift build`."

to the following (or whatever better to you):

"`cargo` not found on the system. Please install Rust (https://rustup.rs/) to enable automatic builds. Or, you may disable automatic Cargo build by setting `LIBCHEWING_AUTO_BUILD_CARGO=0` and manually compile the Rust library before running `swift build`."