[nickdujay-pplx]

Reverts #217 (e03f0278).

Why

#217 has no mixed-version compatibility step. During a rolling upgrade, sandbox-manager and template-manager are separate DaemonSets, so a node can briefly run a new sandbox-manager next to an old template-manager. The new client requires a scope acknowledgement that the old server never sends. Template resolution on create and on restore then fails with FAILED_PRECONDITION "template lookup scope was not acknowledged" until both pods on that node are upgraded. The reverse pairing (old client, new server) mostly works, but every lookup goes to the catalog instead of the local cache.

We can't make every DaemonSet on a node reach the new version at the same moment, so this break would happen on every deploy that ships #217. We're reverting so the next release can go out safely. The change will come back in two phases:

1. Compatible: servers always acknowledge a scope when one is sent and still accept requests without one. Clients send a scope, and if the acknowledgement is missing they log it, emit a metric, and fall back to today's behavior.
2. Strict: once that metric reads zero across the fleet, make the acknowledgement required.

Conflict resolution

The revert conflicted with later changes, and those later changes are kept:

• [template-mgr] record the on-demand pull metric when materializing a resolved release #612 (on-demand pull metrics): the PullSpec and resolve_elapsed materialize signature and the requested_hash metric tagging stay. Only the scope plumbing and the local_execution cache gate are removed.
• [sandbox-manager] materialize an evacuated stopped sandbox's disk artifact without booting it #543 (stopped materialization): its snapshot_restore tests stay. Only the Preserve authenticated scope in cached template lookups #217 restore_template_lookup_uses_validated_owner test is removed.
• claim_create_target and release_create_target in the direct substrate are removed together with their Preserve authenticated scope in cached template lookups #217 call sites. The test for them that Bind the last substrate literals and forbid new ones with archcheck #611 added in substrate_scope.rs is removed as well.
• Cargo.lock: the rstest dev-dependency that Preserve authenticated scope in cached template lookups #217 added to pplx-space-sbx-mgr is removed.

Testing

I didn't build this locally. CI is the compile and test gate.

AI assistance: Perplexity Computer prepared this revert and resolved the conflicts. A maintainer reviews it before merge.