Fix disregard_locked_content guard, make timestamp tests deterministic, add solve-group and channel-extended coverage

- Restore the `should_disregard_conda` check in outdated.rs that was
  accidentally removed, causing ChannelsExtended and pypi-only unsat
  variants to skip per-platform satisfiability (losing validated
  source timestamps).
- Extract `merge_source_timestamp_hints` into a static method with
  unit tests for distinct-variant and conflict-dropping behaviour.
- Replace all `thread::sleep` calls in timestamp tests with
  deterministic host-dep timestamps via `PackageBuilder::with_timestamp`.
  The explicit-update test rewrites mock repodata to publish a newer
  package between locks.
- Add integration test for solve-group environments sharing a source
  package (exercises merge_source_timestamp_hints end-to-end).
- Add integration test for channel-appended (ChannelsExtended) with
  source timestamp preservation.

Author: baszalmstra