[rust] Support subscribing to primary-key table changelog

[fresh-borzoni]

closes #380

Adds changelog (CDC) reads for primary-key tables. Non-append-only ARROW batches carry a per-record change-type vector before the Arrow IPC payload, so the decoder now reads it, and the record scanner no longer rejects PK tables. Create_log_scanner returns the changelog with per-record +I/-U/+U/-D.

The Arrow batch scanner still rejects PK tables (a record batch has no slot for a per-record op), the same as the Java client. A -U/+U pair stays within one poll, also matching Java.

[fresh-borzoni]

@luoyuxia PTAL 🙏

[Copilot]

Pull request overview

Adds CDC (changelog) subscription support for primary-key (KV) tables across the Rust client and language bindings by decoding a per-record ChangeType vector in non-append-only ARROW log batches, and updating scanner validation/docs/examples accordingly.

Changes:

• Decode per-record ChangeType bytes for non-append-only ARROW log batches and propagate them through record-mode scanning.
• Allow record-mode create_log_scanner() on primary-key tables while keeping Arrow batch scanners restricted to log tables; keep -U/+U pairs within a single poll batch.
• Add end-to-end tests plus Rust/Python/C++ docs, examples, and API reference updates for KV changelog scanning.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
website/docs/user-guide/rust/example/primary-key-tables.md	Adds Rust user-guide example for subscribing to PK-table CDC changelog.
website/docs/user-guide/rust/api-reference.md	Documents create_log_scanner CDC behavior and adds ChangeType section.
website/docs/user-guide/python/example/primary-key-tables.md	Adds Python user-guide example for PK-table changelog scanning.
website/docs/user-guide/python/api-reference.md	Documents Python scanner behavior differences for PK vs log tables.
website/docs/user-guide/cpp/example/primary-key-tables.md	Adds C++ user-guide example for PK-table changelog scanning.
website/docs/user-guide/cpp/api-reference.md	Documents C++ scanner behavior differences for PK vs log tables.
crates/fluss/tests/test_fluss.rs	Registers new KV changelog integration test module.
crates/fluss/tests/integration/kv_changelog.rs	Adds end-to-end Rust integration test for PK-table CDC changelog.
crates/fluss/src/record/arrow.rs	Implements changelog wire decoding (attributes bit + per-record change types) and adds unit tests.
crates/fluss/src/client/table/scanner.rs	Allows record-mode log scanning for PK tables; improves validation and error messages.
crates/fluss/src/client/table/log_fetch_buffer.rs	Ensures -U/+U update pairs are kept together within a single poll batch; adds test.
crates/examples/src/example_kv_changelog.rs	Adds Rust example program demonstrating PK-table CDC changelog scanning.
crates/examples/Cargo.toml	Registers the new Rust example target.
bindings/python/test/test_kv_changelog.py	Adds Python integration tests for PK-table changelog scanning and batch-scan rejection.
bindings/python/fluss/init.pyi	Updates Python type stubs/docs for PK-table changelog scanning.
bindings/python/example/kv_changelog.py	Adds Python example for PK-table CDC changelog scanning.
bindings/cpp/test/test_kv_changelog.cpp	Adds C++ integration tests for PK-table changelog scanning and batch-scan rejection.
bindings/cpp/include/fluss.hpp	Updates C++ API docs for PK-table changelog scanning and batch-scan restriction.
bindings/cpp/examples/kv_changelog_example.cpp	Adds C++ example for PK-table CDC changelog scanning.
bindings/cpp/CMakeLists.txt	Builds the new C++ example executable.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[fresh-borzoni]

@luoyuxia Ty, addressed copilot comments, PTAL

[luoyuxia]

@fresh-borzoni Thanks for the pr. LGTM