v0.19.0

CI: add --timestamp=none to App Store codesign to stop 60m hangs

The build-mac-appstore job's codesign commands contacted
timestamp.apple.com (codesign's default), which intermittently stalls
from GitHub macOS runners and timed out the job at 60 minutes. The
comment above the commands already documented --timestamp=none as the
fix (App Store re-signs on ingestion, so RFC3161 timestamps are not
needed) but the flag was never applied. Add it to all three signing
invocations.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>