Fix a crash adding several files at once, from #535

[kddlb]

Summary

#535 moved the C function urlForPath() from PlaylistEntry.m into CogPlaylist, as the Swift PlaylistEntry.url(forPath:). But two files declared the C function themselves and still called it:

• PlaylistLoader.m, in loadInfoForEntries:;
• SQLiteStore.m, in its import of the old library.

The Cog target links with -undefined dynamic_lookup, so the build passed. The call fails only when it's reached, and dyld aborts the app. loadInfoForEntries: is the background metadata load: adding several files at once loads the first in the foreground and the rest there, so adding more than one file at a time crashes main today. My testing of #535 added files one at a time and from cue sheets, neither of which reaches it. Sorry about that.

The fix exposes the Swift version to Objective-C as +[PlaylistEntry urlForPath:]; both callers use it, and the stale declarations go.

Testing

• The app builds. nm -u on Cog.debug.dylib no longer lists _urlForPath.
• Compared with a build from just before CogPlaylist, step 1: the framework, and PlaylistEntry's shared logic in Swift #535, the only new undefined non-Swift symbols are PlaylistEntry and PlaylistStore's classes, and CogPlaylist.framework exports both. Nothing else dangles.
• A new test calls urlForPath: the way Objective-C does. All 28 CogPlaylist tests pass.
• Not yet run in the app: adding several files at once is the check.

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