chore: prepare the repository for v0.3.0

[zjw1111]

Purpose

Prepare the repository and release tooling for Apache Paimon C++ v0.3.0.

• Align version, build, cache, benchmark, feature, and release documentation with the current implementation.
• Refresh ASF checks and bundled-attribution material.
• Fix RC verification to call the build entry point with its supported named arguments and reject nonzero or unparseable Apache RAT unknown-license counts.
• Provision the Rust, HTTP, and TLS build prerequisites in the RC verification workflow.
• Require newly generated release-signing RSA keys to be at least 4096 bits.
• Make SpecialFieldIds constants safe for external ODR-use through C++17 inline constexpr definitions and cover that usage in the install smoke test.
• Use “Apache Paimon C++” for the website title, logo text, and home-page heading.

Linked issue: None.

Tests

• python3 -m unittest scripts.releasing.tests.test_release_tools (13 tests)
• bash -n scripts/releasing/verify_release_candidate.sh
• pre-commit run --files <changed-files>
• git diff --check
• Compiled, linked, and ran the install consumer smoke test against an installed libpaimon.
• Built the Sphinx documentation in an isolated Python 3.12 environment and verified the generated title and heading.
• Ran Apache RAT 0.16.1 against the v0.3.0 source archive.

API and Format

The public SpecialFieldIds integer constants are now C++17 inline static constexpr definitions so external ODR-use links correctly. There is no storage-format or protocol change.

Documentation

Updates contributor, build, release, user-guide, API, and website documentation. The website identity is consistently “Apache Paimon C++”, and release-key guidance now requires 4096-bit RSA keys.

Generative AI tooling

Generated-by: Codex (GPT-5)

[lxy-9602]

+1

[SteNicholas]

LGTM.