The override was landing two of the three defects that keep this client off any
platform but Apple. The third is in the same file: mimeType(for:)'s
pre-macOS-11 branch calls four MobileCoreServices C functions with no guard, and
Linux never reaches that branch — #available is true for a platform it does not
name — but still has to compile it. Five errors, after the imports were fixed.
Guarded, `swift build` completes cold on linux: 2667 files, and the flows the
client ships build and run against it (HTTP 200, 520 models). All three
corrections are no-ops on the four platforms Package.swift declares, so the
Apple output stays byte-identical to the jar's.
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