chore(spm): switch XCFramework releases to cargo-swift

[birdmanmandbir]

Summary

• Replace the custom XCFramework release path with cargo-swift packaging.
• Keep a single SwiftPM product, TaskChampionFFI, backed by TaskChampionCore.xcframework.zip.
• Select linkage from the tag name: snapshot or dynamic tags build dynamic; normal release tags build static.
• Add main-branch snapshot releases with tags like v3.0.2-guion.56-snapshot.20260710123045.abcdef0.
• Remove the legacy doubled-FFI generated header/modulemap and old custom build script.

User-facing workflow

• Formal releases use normal tags and publish static artifacts.
• Main merges publish prerelease dynamic snapshots.
• iOS consumers always add the same product, TaskChampionFFI, and only change the pinned tag.

Verification

• bash -n scripts/package_cargo_swift.sh scripts/use_local_xcframework.sh
• yq . .github/workflows/release-xcframework.yml >/dev/null
• git diff --check
• checked that no FFIFFI text remains in workflow/package/script/source paths