feat(p5): add bounded GCS artifact ports

[Pigbibi]

Summary

• add injected, exact-key GCS adapters for bounded P5 snapshots and create-only receipt admissions
• use generation-match creation; never list, overwrite, delete, configure credentials, schedule work, or contact a broker
• convert storage-port failures into PARKED/receipt_store_unavailable
• document the still-undeployed boundary and add unit coverage for missing, malformed, unavailable, and collision paths

Verification

• PYTHONPATH=src PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest -q (37 passed)
• python -m compileall -q src
• git diff --check

No cloud APIs, buckets, schedulers, paper/live connections, or orders are created by this PR.