chore: prepare release 0.4.0

[shyjsarah]

Purpose

Prepare the repository for the Apache Paimon Vector Index 0.4.0 release line. This is a release-readiness change only; it does not create, sign, or publish release artifacts.

Changes

• align ASF branch protection with the release-0.4 branch naming convention
• harden the release branch/version helper scripts, including Cargo lockfile refresh and local crate dependency version updates
• add automated tests for the release helper scripts and run them in CI
• refresh generated Rust dependency reports from internal version 0.3.0 to 0.4.0
• document the completed 0.3.0 release and move the upcoming release section to 0.4.0

Verification

• python3.11 -m unittest discover -s tools/tests -p 'test_*.py' -v
• python3 tools/validate_asf_yaml.py
• python3 tools/check_license_headers.py
• dependency license checks/report generation and binary legal inventory
• cargo deny check licenses
• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• Rust workspace tests (core, storage fixtures, FFI, and JNI)
• release workspace build
• C and C++ tests
• Java/JNI Maven tests on JDK 8, bundled JNI JAR verification, and loader smoke test
• Python tests, wheel build/content verification, installation, and import smoke test
• cargo publish -p paimon-vindex-core --dry-run
• two source archives generated from the same commit are byte-for-byte identical and contain the required release files
• fork CI passed all jobs on commit 0fff6387ffd18cc6c751979d45b8abd4b27d42b5

[jerry-024]

+1