fix: support modern Swift release builds

[steipete]

Summary

• resolve SwiftPM's actual binary output directory instead of reconstructing the legacy path
• preserve swift-bridge entry points in optimized Swift builds
• reuse one Swift build command for compilation and output-path discovery

Why

Current Swift toolchains can place products below Products/Debug or Products/Release, so the hard-coded <triple>/<profile> link path fails. Optimized builds can also strip the internal @_cdecl bridge functions generated by swift-bridge; this is tracked in swift-bridge#166.

Validation

• cargo fmt --check
• cargo check
• optimized macOS Tauri app bundle build with Swift 6.4
• verified the release static library exports the generated bridge symbols

[Copilot]

Pull request overview

This PR updates the macOS Swift build step in build.rs to be compatible with modern SwiftPM output layouts and to prevent swift-bridge-generated entry points from being stripped in optimized Swift builds.

Changes:

• Switch from reconstructing the SwiftPM output directory to querying it via swift build --show-bin-path.
• Preserve swift-bridge entry points in release builds by adding -Xswiftc -enable-testing.
• Refactor the Swift invocation into a shared swift_build_command() helper and return the resolved build output directory for linking.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.48%. Comparing base (2a00e01) to head (35d04a6).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #269   +/-   ##
=========================================
  Coverage     50.48%   50.48%           
  Complexity       65       65           
=========================================
  Files            31       31           
  Lines          6499     6499           
  Branches        235      235           
=========================================
  Hits           3281     3281           
  Misses         3184     3184           
  Partials         34       34           

Flag	Coverage Δ
android	27.98% <ø> (ø)
ios	78.76% <ø> (ø)
javascript	92.10% <ø> (ø)
macos	64.59% <ø> (ø)
rust	35.02% <ø> (ø)
windows	15.18% <ø> (ø)

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.