[codex] Add Rust and WASM HTML API implementation

[sirreal]

Summary

Adds a Rust-backed HTML API prototype under ext/html-api-rust with native PHP extension and WASM package entry points. The Rust core builds as a shared native/WASM library, while the JavaScript wrapper exposes WordPress-like HTML API classes for browser and Node usage.

What changed

• Adds the Rust HTML API core, native PHP extension shim, build scripts, and documentation.
• Adds the WASM/NPM package wrapper with TypeScript declarations, loader support, browser smoke coverage, package artifact checks, and html5lib tree-test coverage.
• Updates the PHP HTML API classes for native/WASM parity work around processor and tag processor behavior.

Validation

• npm --prefix ext/html-api-rust/wasm run build
• npm --prefix ext/html-api-rust/wasm run test:all
• npm --prefix ext/html-api-rust/wasm run test:browser

The html5lib tree runner completed with zero unsupported cases, aside from the known WordPress duplicate shell-attribute skips documented in the package tests.