Build xberg PHP extension alongside kreuzberg in 8.5 image

[nikola-jovanovic-php]

📖 Context

Kreuzberg has been renamed/superseded by Xberg (migration guide); v4 is LTS-only until end of 2026. The 8.5/Dockerfile builds the kreuzberg PHP extension from Rust source (kreuzberg-builder stage, KREUZBERG_VERSION=v4.9.8) and ships it commented out in php.ini for consumers to enable.

api.bang.com needs ext-xberg in this base image before it can migrate off Kreuzberg v4 LTS: sctr/api.bang.com#5235.

🎯 Goals

• Add an xberg-builder stage: clone xberg-io/xberg at a pinned tag, build the PHP extension crate, ship the xberg extension commented out in php.ini
• Keep the kreuzberg extension available during the transition so consumers can switch independently; remove it once downstream migrations complete
• Verify the extension crate path and .so name in the xberg repo (v4 was crates/kreuzberg-php → libkreuzberg_php.so)

⚙️ Technical

• Touchpoint: 8.5/Dockerfile (kreuzberg-builder stage, ~lines 17–51)
• ⚠️ Latest Xberg release is currently a v1.0.0-rc.x pre-release — pin deliberately or wait for a stable tag