Post-review deferred items (from #9/#11 review)

Deferred non-blocking findings from the #9/#11 review, batched:

- [ ] `SandboxTemplate` hardcodes the `gvisor` toleration — parameterize the
tier-owned scheduling fields (toleration + nodeSelector) from
`sandbox.runtimeClassName` per the ADR-0007 extension contract, with a
rendered-fixture test proving a values-only new tier schedules correctly
- [ ] Conformance script: run in a dedicated `conformance-${RUN_ID}` namespace
with run-scoped PVC names + cleanup trap; add a gVisor × NetworkPolicy
combined probe (allowed 443 + denied non-443 under the gvisor RuntimeClass)
- [ ] Amazon VPC CNI add-on: select an explicitly supported version for the
declared EKS version and document the upgrade procedure (currently floats)
- [ ] Fix EKS version drift in comments (1.34 → 1.35)
- [ ] Evaluate `NETWORK_POLICY_ENFORCING_MODE=strict` with enumerated
startup flows and rollback criteria (ADR-0008 startup-window caveat)

Start with the ADR-0007 extension contract, ADR-0008 startup-window caveat, the SandboxTemplate, and the conformance script. Work through each deferred finding separately, including the rendered-fixture test and gVisor × NetworkPolicy probe; done requires all listed scheduling, namespace cleanup, version, documentation, and rollback criteria items to be addressed.

Written by the indexing model from the issue text.