fix(shared-core): keep symlinks in the macOS XCFramework zip

[bmc08gt]

KMMBridge's zipXCFramework task is a bare Gradle Zip task, and Gradle's Zip/CopySpec machinery dereferences symlinks instead of storing them. A macOS framework bundle is built entirely from symlinks — Versions/Current -> A, and the top-level Headers/Modules/Resources/binary each symlinking into Versions/Current/* — so the archive KMMBridge produces for the macOS slice ships three literal duplicate copies of the same content instead of the real bundle layout. iOS framework bundles are flat, which is why this was invisible until the macOS targets landed (#1402).

The published 0.5.0 release confirms it: unzipping the release asset and listing the macOS slice shows Versions/Current, Headers, Modules, Resources, and the binary as full duplicated files rather than symlinks. Any consumer that builds and codesigns against the macOS slice fails with code object is not signed at all on the nested framework, because codesign's recursive bundle signing needs the real symlinked structure.

KMMBridge 1.2.1 is still the newest release (checked the Gradle Plugin Portal and GitHub releases) and its issue tracker has nothing open about this, so there is no upstream fix to pick up. This reconfigures the existing zipXCFramework task: once KMMBridge's own zip runs, doLast deletes the result and re-zips the same source directory with zip -y, which stores symlinks as symlinks instead of following them. The reconfiguration only touches the task when KMMBridge registers it (matching {}), so it stays a no-op on any build where publishing is not enabled.

Verified locally by running :kmp:shared-core:zipXCFramework directly (placeholder local.properties and spmRepoDir, no real publish) and extracting the resulting archive: the macOS slice now has real symlinks (Headers -> Versions/Current/Headers, Current -> A, etc.) instead of duplicated files.

This unblocks a 0.5.1 release of flipcash-shared-core-spm — needed before code-ios-app#725 can adopt shared mnemonic derivation against a macOS-capable build.