Polish: honest consistency docs, hardened failure demo, join/leave + CI

[tomlin7]

PR #1 already merged. This is merge-ready polish on that MVP — no rewrite, no AI.

Why

The merged demo grepped the whole /ring JSON for "node-b". That string is always present in the cluster nodes list, so the “key owned by node-b” picker could pick a key that node-b does not own.

What changed

• Failure demo parses owners[] only. Two scenarios:
  • A: SET acked by both owners, kill one replica → GET still hits
  • B: SET while an owner is down (acked=1), kill the surviving copy → GET misses
• Consistency is the README lead: success is acked≥1, not linearizable; R=2 sync fan-out; last-writer-wins; no persistence
• 60-second path: ./scripts/quickstart.sh (or make cluster + curl)
• Join/leave: POST/DELETE /admin/members (local, not gossip) + ./scripts/rebalance-demo.sh (~1/N remap)
• CI: gofmt, vet, -race, then both demo scripts against a local 3-node cluster
• Small correctness nits: read-repair no longer cancels its own RPCs; Store.Close is idempotent

How to run (60s)

./scripts/quickstart.sh
./scripts/failure-demo.sh
./scripts/rebalance-demo.sh
make stop

Verified on this agent VM (2026-09-13)

• make ci (gofmt, go vet, go test ./... -race)
• ./scripts/quickstart.sh — SET user:1 acked 2/2, GET via node-b hit
• Failure demo A: demo:failure-a:1 owners [node-b, node-a], SET acked 2/2; after kill, :8080/:8082 served node-a; :8081 down
• Failure demo B: demo:failure-b:1 owners [node-b, node-a]; SET while node-b down → acked=1 failed=[node-b]; kill node-a → GET :8082 503
• Rebalance: 617/2000 = 0.308 primaries remapped on join (N=3→4); leave restored 2000/2000
• Bench (loopback, this commit, 2026-09-13T10:55:13Z, 4 vCPU Xeon): SET 9395 ops/s p99 8.90ms; GET 10583 ops/s p99 11.31ms. A quieter same-day slot was ~12.8k / ~31k — both labeled, neither is an SLO
• Docker was not available on this VM; compose path not re-run here

Honesty

Best-effort sync replication. Success is acked≥1. Last-writer-wins. Static-unless-you-POST membership. No persistence.