feat(deltagraph): server binary backed by Databricks (Phase 4.1)

[genezhang]

Summary

Ships Phase 4.1 of the DeltaGraph plan — a deltagraph binary alongside clickgraph, both built from the same library. The new binary defaults --databricks on, defaults Neo4j compat mode on (since the headline demo is open Neo4j Browser, run Cypher, hit Databricks under the covers), and forces a token-from-env policy (DATABRICKS_TOKEN is never accepted on the command line).

Architecture: one library, two binaries:

• ServerConfig.databricks: bool sits alongside ServerConfig.embedded with CLICKGRAPH_DATABRICKS env support. A new feature-gated branch in server::run_with_config builds a DatabricksSqlExecutor (from Phase 2.1) and routes it through the existing QueryExecutor trait. AppState.clickhouse_client stays None — only DDL writes consume it, and the Databricks read path blocks writes upstream via write_guard already.
• New [[bin]] required-features = [\"databricks\"] keeps the default cargo build producing only clickgraph — no reqwest/Databricks dep pulled into the standard build.
• clickgraph also picks up a --databricks flag for parity (users on a --features databricks build can flip dialects without two binaries).

Build/run:

cargo build --release --features databricks --bin deltagraph
export DATABRICKS_HOST=dbc-abc123-def4.cloud.databricks.com
export DATABRICKS_WAREHOUSE_ID=abc123def456
export DATABRICKS_TOKEN=dapi-...
./target/release/deltagraph

Tests

• tests/rust/bin/deltagraph_smoke.rs (gated on feature = \"databricks\", two assert_cmd tests):
  • deltagraph_help_lists_databricks_specific_env_vars — --help mentions DATABRICKS_HOST/WAREHOUSE_ID/TOKEN and the env-only policy. Pins the user's discovery surface against accidental help-text rewrites.
  • deltagraph_startup_without_credentials_errors_clearly — missing-creds startup exits non-zero with a precise error naming the missing field (DATABRICKS_HOST not set), not a confusing reqwest connection error.

$ cargo build --bin clickgraph                                  # default build, no databricks dep
$ cargo build --bin clickgraph --features databricks            # adds --databricks flag to clickgraph
$ cargo build --bin deltagraph --features databricks            # produces the new bin
$ cargo test --features databricks --test deltagraph_bin        # 2/2 smoke tests pass
$ cargo test -p clickgraph --features databricks --lib          # 1385 lib tests pass
$ cargo clippy --all-targets --features databricks              # 0 warnings
$ cargo fmt --all --check                                       # clean

What's deferred to Phase 4.3

A full "Bolt → DeltaGraph → Databricks wiremock" startup integration test belongs in Phase 4.3 (the headline Neo4j Browser e2e demo), where it can drive an actual Cypher query through the live server stack. Spinning up the HTTP + Bolt server for a single request in this PR would be expensive scaffolding for a different feature.

Test plan

• cargo build --bin clickgraph (default features) — unchanged
• cargo build --bin deltagraph --features databricks — produces working binary
• cargo test --features databricks --test deltagraph_bin — 2 smoke tests pass
• cargo test -p clickgraph --features databricks --lib — 1385 tests pass
• cargo clippy --all-targets --features databricks — 0 warnings
• cargo fmt --all --check — clean
• STATUS.md mentions the new deltagraph binary

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a Databricks-backed server distribution (“DeltaGraph”) by introducing a feature-gated deltagraph binary that reuses the existing server library and routes execution through the Databricks SQL Warehouse executor while keeping the default clickgraph build unchanged.

Changes:

• Introduces a new deltagraph binary (behind --features databricks) with Databricks-first defaults and env-only token policy.
• Extends shared server configuration and startup routing to support a Databricks execution backend (--databricks flag + CLICKGRAPH_DATABRICKS env).
• Adds feature-gated smoke tests validating deltagraph --help discovery strings and clear missing-credentials startup errors.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/rust/bin/deltagraph_smoke.rs	Adds Databricks-feature-gated smoke tests for the new deltagraph binary help text and missing-credentials failure path.
STATUS.md	Documents the new deltagraph server binary as part of DeltaGraph / Databricks mode.
src/server/mod.rs	Adds a databricks startup branch to build a Databricks executor and run the server without a ClickHouse client; adds env-based Databricks config builder.
src/main.rs	Adds --databricks CLI flag to clickgraph for parity (feature-gated at runtime).
src/config.rs	Extends ServerConfig / CliConfig with a databricks: bool flag and env parsing (CLICKGRAPH_DATABRICKS).
src/bin/deltagraph.rs	Implements the new deltagraph binary wrapper that forces Databricks mode and defaults Neo4j compat on.
Cargo.toml	Declares deltagraph as a gated [[bin]], adds dev-deps for smoke tests, and registers the standalone test target behind databricks.
Cargo.lock	Locks new dev-dependencies (assert_cmd, predicates) into the workspace.