[samgbafa]

TC-887. Draft, not for merging. This PR prepares the deploy for an at-rest root from the environment (base: #515). It stays a draft until the switch-on prerequisites tracked on TC-887 are met, and it will be rebased onto the top of the at-rest stack before then.

What

• docker-compose.dstack-postgres.yaml:
  • adds TINYCLOUD_AT_REST__ROOT_SOURCE: env and TINYCLOUD_AT_REST__MODE: "off" as literals in the measured compose, and TINYCLOUD_AT_REST_ROOT_KEY: "${AT_REST_ROOT_KEY:?…}" from the sealed environment;
  • the trust-bundle comment becomes ASCII-only, because the node reads the measured compose with a strict, printable-ASCII parser.
• With mode = "off" the node provisions and checks the keys (fingerprint in /at-rest/status) without encrypting. Turning encryption on is a later, reviewed one-line change to "encrypt".
• docker.yml deploy:
  • maps the new production secret PROD_TINYCLOUD_AT_REST_ROOT_KEY as a required input;
  • refuses a value that isn't ^[A-Za-z0-9+/]{43}=$ with a constant message, before .phala-prod.env is written;
  • seals it as AT_REST_ROOT_KEY;
  • the preflight passes it, with ROOT_SOURCE=env, to --validate-config (feat(TC-887): node wiring for at-rest keys, with a root from the environment #515 makes that check the key).
• The N0 template hash (6cc584f6…, raw eef0594d…), its byte-identical fixture scripts/ci/fixtures/tc959-n0-compose.yaml, docs/releasing.md and CHANGELOG move together.
• Tests:
  • scripts/ci/test-deploy-recovery.py deploys with a valid key, checks the key never reaches logs, and adds a refusal test for malformed keys;
  • the escrow crate's production_compose_passes is back;
  • a node unit test checks that the production compose passes the env-root boot check.

Before this can deploy

• The node refuses root_source = "env" in a TEE while the measured app-compose allows DSTACK_AUTHORIZED_KEYS, or on a non-production OS image (feat(TC-887): node wiring for at-rest keys, with a root from the environment #515). The SSH-key provisioning and the image must change first.
• The secret must exist in the production environment. Once AT_REST_ROOT_KEY is in the CVM's allowed_envs, it must stay present on every deploy and rollback.

Verification

• python3 scripts/ci/test-deploy-recovery.py: 31 tests OK.
• bash scripts/ci/test-phala-os-guard.sh: passed.
• cargo test --locked -p tinycloud-at-rest-escrow production_compose
• cargo test --locked -p tinycloud-node --lib -- at_rest::tests