docs: dn_redis is a key-shape protocol + Rust command type model

docs/DN_REDIS_KEY_SHAPE_PROTOCOL.md

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+# dn_redis is a key-shape protocol + Rust command type model — not a Redis service dep
+
+## TL;DR
+
+`lance-graph-cognitive::container_bs::dn_redis` is a **key-shape protocol definition + a Rust-side command type model** for HHTL-keyed hot lookups. It is NOT:
+
+- A network-protocol implementation (no RESP encoder/parser, no listener, no command executor)
+- A drop-in replacement that lets existing Redis clients talk to lance-graph
+- A wire-protocol emulator (the FalkorDB / KùzuDB precedent does NOT apply here — those projects implement the Redis RESP protocol; dn_redis does not)
+
+What dn_redis IS:
+
+- A **Rust API** that exports key-construction helpers (`dn_key`, `spine_key`, `walk_to_root_keys`, `children_pattern`, `subtree_pattern`) producing `String` keys like `ada:dn:{hex}` and `ada:spine:{hex}`
+- A **command type model** (`RedisCommand` enum + `RedisPipeline` struct) that adopters can populate to describe the operations they need to execute
+- A **serde layer** for `CogRecord` payloads (`cog_record_to_bytes`, `cog_record_from_bytes`)
+
+## What is missing — and what adopters have to provide
+
+The module exports the SHAPE of the operations; adopters provide the EXECUTOR. There is no `Backend` trait, no listener, no `connect()` function, no `execute()` method on `RedisPipeline` — those are what consumer code must implement.
+
+To use dn_redis, a consumer must:
+
+1. Decide on a backend (see below)
+2. Write an executor that takes `RedisPipeline` (or individual `RedisCommand` values) and runs them against the chosen backend
+3. Mount the executor at the call sites where the consumer's cognitive substrate needs hot-key lookups
+
+This doc proposes formalizing two valid backend categories so adopters do not have to discover them by reverse-engineering the call sites.
+
+## Two valid backend categories
+
+### Backend A: standalone Redis (or Redis with hash-tag re-keying)
+
+A consumer can run a Redis server and pipe `RedisCommand`s to it via any Redis client (`redis-rs`, `fred`, etc.). The consumer writes the executor: receive `RedisCommand`, translate to the client's API, return results.
+
+**Important caveat (per codex P2 review on PR #455):** the documented `walk_to_root` operation uses `MGET ada:dn:{ancestor1} ada:dn:{ancestor2} ...`. In a **Redis Cluster** deployment, `MGET` requires all keys to belong to the same hash slot — which means they must share a `{hash_tag}` substring per the [Redis Cluster specification](https://redis.io/docs/latest/operate/oss_and_stack/reference/cluster-spec/). The current key layout `ada:dn:{hex}` does NOT include a hash tag, so cluster `MGET` will return `CROSSSLOT Keys in request don't hash to the same slot` errors.
+
+For Redis Cluster to be a valid backend, EITHER:
+- (i) The key layout must be re-shaped to include a shared hash tag for keys that are co-queried (e.g. `ada:dn:{root_basin}:0102...` so all descendants of a basin hash to one slot)
+- (ii) The consumer must split multi-key operations into per-key calls (defeats the pipeline)
+- (iii) The consumer runs standalone Redis (not Cluster)
+
+This caveat is the practical reason most consumers should treat dn_redis's key shape as "designed for standalone Redis OR for an in-binary executor", not for a clustered Redis deployment without re-shaping.
+
+### Backend B: in-binary executor over Lance via DataFusion
+
+A consumer can implement an executor that takes the same `RedisCommand` shape and runs it against the local Lance dataset via DataFusion queries. The result is Redis-protocol-shaped responses (via the Rust types — not wire-protocol bytes) emitted from the consumer's own data, with **no Redis service required**.
+
+What the consumer writes (per codex P2 #3 — there is no shipped trait to implement; this IS new consumer code):
+
+```rust
+// Consumer code, NOT lance-graph code
+struct LanceBackend { /* ... */ }
+
+impl LanceBackend {
+    fn execute(&self, cmd: RedisCommand) -> Result<RedisValue, Error> {
+        match cmd {
+            RedisCommand::Get(key) => {
+                let dn = self.parse_dn_from_key(&key)?;
+                let row = self.lance.read_by_dn(dn).await?;
+                Ok(RedisValue::Bulk(cog_record_to_bytes(&row)))
+            }
+            RedisCommand::Mget(keys) => { /* batch lookup over Lance */ }
+            RedisCommand::Keys(pattern) => { /* DataFusion scan filtered by prefix */ }
+            // ... etc per the enum's variants
+        }
+    }
+}
+```
+
+This is "Redis-shape over Lance" — the consumer projects Lance results through the command-result type. There is no protocol parsing because no wire protocol is involved; everything is in-process Rust types.
+
+## What this doc does NOT claim
+
+- **It does not claim FalkorDB or KùzuDB use dn_redis.** Those are independent products that implement the actual Redis RESP wire protocol; dn_redis does not implement RESP. The earlier version of this doc cited them as a precedent for "talk Redis without being Redis" — that framing was wrong. The honest framing is more constrained: dn_redis provides the key-shape and command-type contract that a consumer can EITHER pipe to a real Redis (standalone) OR execute in-binary against their own data.
+- **It does not document a network protocol.** There is no listener, parser, or executor shipped by lance-graph for dn_redis. Calling it "wire-protocol emulation" (as a prior version of this doc did) misleads consumers about what is implemented vs what they must implement themselves.
+- **It does not claim Redis Cluster is plug-and-play.** Per the caveat above, the current key shape is incompatible with cluster `MGET` slot routing.
+
+## Why this matters
+
+For single-binary deployments (e.g. `AdaWorldAPI/bardioc/substrate-b`) the natural backend is option (B): an in-binary executor over Lance. This means "zero application-level boundaries within substrate-b" (per the PR #452 / #454 append-only-Raft doc) survives the addition of HHTL-keyed hot lookups precisely because dn_redis is a TYPE MODEL, not a service dependency.
+
+For consumers who already operate a standalone Redis (with no clustering or with cluster + re-keyed hash tags), option (A) is straightforward: write the executor that translates `RedisCommand` to a Redis client's API.
+
+The earlier version of this doc framed dn_redis as wire-protocol emulation; codex review on PR #455 caught the inaccuracy. The corrected framing is more constrained but more honest: dn_redis is the SHAPE, the consumer provides the EXECUTION.
+
+## Cross-references
+
+- `lance-graph-cognitive::container_bs::dn_redis` — the key-shape protocol + command type model
+- `lab-vs-canonical-surface.md` — the canonical-vs-lab discipline that frames adapters
+- Companion docs `APPEND_ONLY_RAFT_DOVETAIL.md` + `CLUSTER_ASYMMETRY.md` (PR #452 / #453 / #454 — merged)
+- `AdaWorldAPI/bardioc` PR #15 conversation thread (where this doc + the corrections originated)
+- [Redis Cluster specification](https://redis.io/docs/latest/operate/oss_and_stack/reference/cluster-spec/) for the hash-slot constraint