bundle-qmd.sh signs qmd/qmd and vec0.dylib with Developer ID + hardened
runtime, but the certificate wasn't in the keychain yet when it ran
(Tauri imports the cert internally, AFTER beforeBuildCommand completes).
Fix: add explicit 'Import Apple Developer certificate' step before the
Tauri build step, using security create-keychain + security import.
This makes the cert available to codesign during beforeBuildCommand.
aaecd9a626