[P0-23] Implement the initial Valkey target-lease backend with atomic fencing scripts

[lost-rob0t]

Decision

Use Valkey as the first deployable KV backend behind the lease-store protocol. Keep the protocol backend-neutral so a stronger-consistency backend can be added without changing target or HTTP code.

Required implementation

• One atomic acquire script: create lease only when free/expired and increment a monotonic fencing counter.
• Atomic renew script requiring matching lease id, owner, and fencing token.
• Atomic compare-and-delete release.
• TTL stored by the backend; server time/TTL result is authoritative.
• Request-id result cache for idempotent retries.
• Namespaced keys suitable for standalone and clustered deployments.
• Connection pooling, authentication, TLS configuration, deadlines, reconnect/backoff, and health checks.
• No use of unsafe GET then SET races.
• Document failover/replication assumptions and where fencing is enforced downstream.

Acceptance tests

• Backend contract suite passes against a real Valkey service.
• 100 concurrent acquire attempts yield one owner and one fencing token generation.
• Expiry followed by reacquire produces a strictly larger token.
• Old owner cannot renew, release, or commit after reacquire.
• Script interruption/retry is idempotent by request id.
• Credentials and lease metadata are redacted from logs where required.

[lost-rob0t]

Acceptance complete. PR #95 merged into canonical master as 7834d6614378e01232a1c810249d9ae83cec25c5 from verified head 652b148c233646af4f2e4b46dd6289ae73ef3c26.

Current-head evidence before merge:

• nix flake check --show-trace — successful in Smoke Tests.
• nix build .#default --no-link --print-build-logs — successful in Smoke Tests.
• nix run .#star-unit-tests — successful in Smoke Tests.
• nix run .#star-integration-tests — successful; the runner boots real authenticated plain + TLS Valkey and runs required suites that fail on zero tests, partial execution, skips, or failures.
• ./scripts/stack-test.sh — successful in Container Stack.
• python -m unittest discover -s tests -p 'test_*.py' -v and git diff --check — successful in Operational salvage.
• Canonical schema lock workflow — successful.

Exact recorded suite counts:

• LEASE-STORE-CONTRACT-TESTS: 8 discovered / 8 executed / 8 passed / 0 failed / 0 skipped.
• VALKEY-LEASE-INTEGRATION-TESTS: 22 / 22 / 22 / 0 / 0.
• COUCHDB-VIEW-INTEGRATION-TESTS: 7 / 7 / 7 / 0 / 0.
• HTTP-API-TESTS: 28 / 28 / 28 / 0 / 0.
• Operational salvage: 12/12.
• Container stack: passed.
• Schema lock: verified.

The final acceptance regressions include atomic per-key list validation across release/reacquire, missing-key PTTL=-2 rejection, and a corrupt unsupported record_version that exercises the Lisp deserialization fail-closed boundary. All review threads were resolved after verifying the current implementation and tests.