feat(four-repo-demo): end-to-end integration demo crate

Wave-5 worker W-INT-1. New excluded crate at `crates/four-repo-demo/`
that demonstrates three of the four glue surfaces working together:

* `cognitive-shader-actor::CognitiveShaderActor` wrapping a real
  `SupervisableShader` (`SumShader`) — running-sum state via
  `Arc<Mutex<i64>>`, payload is `arrow_array::RecordBatch`.
* `lance-graph-contract::actor::SupervisableShader` trait consumed
  end-to-end (impl is real, not stub).
* `lance-graph-contract::ir::{Operator, OperatorTree, EngineHint,
  Cardinality}` — builds a 3-node tree (RangeScan → Filter →
  CognitiveApply) and computes total estimated cardinality.

The example at `examples/run_demo.rs`:
1. Spawns CognitiveShaderActor<SumShader> via ractor 0.15.13
2. Sends two ShaderMessage::Apply with single-row Int64 batches (5, 7)
3. Sends ShaderMessage::Drain
4. Prints the running sum (12) + the operator tree + cardinality estimate

Test results (cargo test --manifest-path crates/four-repo-demo/Cargo.toml):
* sum_shader_actor_running_sum ... ok
* operator_tree_has_three_nodes ... ok
* operator_tree_shape_correct ... ok
* operator_tree_total_cardinality ... ok
* 2 doc-tests pass
* All 6/6 tests pass

NOT demonstrated (and why, in the crate's lib.rs and README):
* Glue #1 surrealdb-ractor — needs a live SurrealDB
* Glue #2 lance-graph-tikv-provider — needs a TiKV cluster
* Glue #3 sea-orm-ractor — needs running Postgres + the derive

Workspace: added to `exclude` (heavy tokio + ractor + arrow deps).
.gitignore: `/crates/four-repo-demo/target/` to keep build artefacts
out of git.

Worker: W-INT-1. Wave-5 of the four-repo integration.

Author: claude

.gitignore

@@ -70,3 +70,4 @@ crates/thinking-engine/data/Qwopus3.5-27B-v3-BF16-silu/token_embd_4096x4096.u8
 
 # Test fixtures (public-domain German legal/medical/tax data)
 .test/
+/crates/four-repo-demo/target/

Cargo.toml

@@ -25,6 +25,11 @@ exclude = [
     # in the workspace. See .claude/plans/integration-plan.md §5 and §6.
     "crates/lance-graph-tikv-provider",
     "crates/cognitive-shader-actor",
+    # End-to-end integration demo (wave-5 W-INT-1). Excluded so the heavy
+    # tokio/ractor/arrow deps don't grow the workspace check; it's a
+    # standalone runnable consumer of the contract + glue crates.
+    "crates/four-repo-demo",
+    "crates/four-repo-demo",
     # Python bindings (upstream-inherited, opt-in via --manifest-path)
     "crates/lance-graph-python",
     "crates/lance-graph-codec-research",