Publish RCs from public Maven repository

[ShawnChen-Sirius]

Summary

• publish RC artifacts as a standard Maven repository for anonymous users at https://maven.chdb.io
• stage immutable Maven repository files in the RC release workflow
• add the Vercel gateway configuration and maintainer upload tooling
• update the user-facing installation documentation for Maven and Gradle

Validation

• actionlint .github/workflows/rc-release.yml
• shellcheck scripts/publish-rc-repository.sh
• bash -n scripts/publish-rc-repository.sh
• git diff --check
• vercel build --prod in deploy/vercel-maven
• resolved the JDBC driver and all four native platform artifacts from a clean Maven project
• verified the published artifacts against their authoritative SHA-256 digests

Note

Publish RC artifacts to public Maven repository at maven.chdb.io

• Replaces GitHub Packages in the RC workflow with local Maven-repository staging: checksum sidecars are generated, metadata files removed, and the repo is uploaded as a workflow artifact for manual Vercel Blob publication.
• Build matrix now produces pure-Java artifacts (parent, JDBC, BOM) only from the linux-x86_64-gnu job; other jobs produce their platform-native JAR only.
• Adds a Vercel Maven gateway (vercel.json + index.html) that rewrites /com/ paths to the Blob store with no-store caching and a nosniff header, plus a landing page.
• Adds publish-rc-repository.sh supporting --dry-run validation, preflight conflict detection (refuses overwrite), per-run rollback on error or HUP/INT/TERM, and public non-overwriting Blob uploads.
• Updates README to point at the anonymous maven.chdb.io repository and removes the GitHub PAT settings example.
• Risk: publication is now manual and immutable — the publisher refuses overwrites and incomplete rollback requires manual cleanup; the BOM example changed to 1.0.0-rc.1.

^{Macroscope summarized 1304292.}