feat(c): expose batch incremental reads

[JingsongLi]

Purpose

Linked issue: N/A

Expose the existing fixed-range incremental scan and Arrow read capabilities through the C ABI. C consumers can now plan (start_exclusive, end_inclusive] snapshot ranges instead of being limited to full batch scans.

Brief change log

• Add C mode constants for Delta, Changelog, Auto, and Diff incremental scans.
• Add opaque incremental scan and plan handles with result and free functions.
• Add incremental planning, work-unit counting, and ranged Arrow reading APIs.
• Preserve ReadBuilder projection and predicates across incremental planning and reading.
• Add ABI signature guards plus behavioral and null-boundary tests.

Tests

• cargo test -p paimon-c
• cargo clippy -p paimon-c --all-targets -- -D warnings
• mkdocs build --strict
• Generated the C header with cbindgen and verified all incremental declarations.

API and Format

This is an additive C ABI change. It does not change the Paimon storage format. The new API performs fixed-range batch incremental reads and does not wait for future snapshots.

Documentation

Updated the C integration guide with mode semantics, a complete lifecycle example, ownership rules, and the fixed-range limitation. Cross-linked the batch incremental reading guide to the new C API.