…b split, part 2]

Part 2 of the Delta Lake contrib PR breakup (tracking: apache#4366). Establishes the
`contrib-delta` build gate and the inert wiring that lets a gated build compile
end to end, while the DEFAULT build stays byte-for-byte unchanged (zero Delta
surface). No real Delta read logic yet -- that lands in later parts; here a Delta
read that reaches native returns a clean "not implemented" error and falls back
to vanilla Spark.

Build gate:
- Maven `contrib-delta` profile (spark/pom.xml) with per-Spark `delta.version`
  (3.5->3.3.2, 4.0->4.0.0, 4.1->4.1.0) and an add-source of contrib/delta/src.
  Default `delta.version` floor in pom.xml. The default spark.version stays 4.1.2
  (the delta-spark 4.1.1 pin is a separate, deferred decision).
- Cargo `contrib-delta` feature on core (optional path dep on comet-contrib-delta);
  `native/Cargo.toml` excludes ../contrib from the workspace.
- `dev/verify-contrib-delta-gate.sh` proves default cargo/mvn/dylib carry zero
  Delta surface and the gated build pulls the right deps; wired into a minimal
  `delta_build_gate.yml` CI job (the full suite/regression workflows land later).
  Hardened the script against a `set -o pipefail` + `grep -q` SIGPIPE misfire
  (early grep exit -> echo SIGPIPE -> false guard failure) via here-strings.

Inert wiring:
- Proto: `Delta*` messages + `delta_scan = 118` (117 is BroadcastNestedLoopJoin).
- Native dispatch: `OpStruct::DeltaScan` arm with a not-compiled-in error on
  default builds and a feature-gated `delta_scan` shim that calls the contrib;
  exhaustive-match arms in operator_registry/jni_api; `convert_spark_types_to_
  arrow_schema` promoted to pub(crate).
- Stub contrib crate (contrib/delta/native): `plan_delta_scan` returns
  `DataFusionError::NotImplemented` -- just enough to satisfy the core shim's
  contract so `--features contrib-delta` links.
- JVM bridge `DeltaIntegration` (reflective, all lookups return None until the
  contrib classes exist), the CometExecRule Delta-marker hook (CDF hook deferred
  to a later part), the CometScanRule Delta delegation + metadata-col reorder,
  and the leaf `DeltaConf`.

Verification: default + gated native build, clippy both feature states, gate
script, gated + default JVM compile, spotless/scalastyle, cargo fmt -- all green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

… + JNI) [Delta contrib split, part 3a]

Part 3a of the Delta Lake contrib PR breakup (tracking: apache#4366). Replaces the build-gate
stub crate's deps with the real driver-side modules and the delta-kernel-rs dependency,
while the executor-side read path stays deferred (the `planner` stub still returns
NotImplemented until part 3b adds `kernel_scan`/`dv_reader` + the real planner).

Driver side (open table, replay log, push predicates, return a DeltaScanTaskList over JNI):
- `error.rs`  - DeltaError / DeltaResult.
- `engine.rs` - delta-kernel engine + object_store config (S3/Azure/GCS/local) for log replay.
- `predicate.rs` - Catalyst -> kernel predicate translation for file skipping.
- `scan.rs`   - log replay -> DeltaFileEntry/DeltaScanPlan; scan-task assembly.
- `jni.rs`    - `Native_planDeltaScan` / `Native_planDeltaReadSchemas` JNI entry points
  (the JVM `Native.scala` that calls them lands in part 4b).
- `lib.rs`    - declares the driver modules + keeps the `planner` stub; drops the
  `dv_reader`/`kernel_scan` module decls (part 3b).
- `Cargo.toml` - only the deps the driver set uses (delta_kernel, object_store, arrow,
  jni, prost, serde_json, url, thiserror, log + jni-bridge); executor deps arrive in 3b.

The driver set is self-contained (`jni -> scan -> engine -> error`, `predicate` standalone);
nothing references the deferred modules. Core is untouched -- the dispatch shim still calls
`planner::plan_delta_scan` (the stub) so a Delta read falls back to vanilla Spark until 3b.
The gate-verify cargo-tree assertion is re-tightened to require `delta_kernel` (now real).

Verification: gated native build, 54 in-crate unit tests (cargo test), default native build
unchanged, clippy (both feature states), gate-verify script, cargo fmt -- all green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>