[PHP] Add Blueprint PHP extension loading step

[adamziel]

What it does

Adds a loadPHPExtension Blueprint step on top of the runtime extension loader from #3566. Blueprints can now load either a direct .so resource or a compile-extension manifest.json resource, plus optional sidecar files through extraFiles.

This is the middle PR in the stack: #3566 runtime loader → this Blueprint step → #3567 compiler tools.

Rationale

The runtime PR gives Playground a shared loadPHPExtension() API, but extenders also need a declarative Blueprint entry point. Keeping this in a stacked PR makes the runtime API reviewable on its own while preserving the same loading path for Blueprint users.

Implementation

The step resolves source as a normal Blueprint file resource, infers sourceFormat from .json vs .so, then calls @php-wasm/universal:

{
  "step": "loadPHPExtension",
  "source": {
    "resource": "url",
    "url": "https://example.com/extensions/example/manifest.json"
  },
  "sourceFormat": "manifest"
}

literal:directory is now recognized as a directory resource so extraFiles can carry data files or dependency assets. URL resources retain a non-enumerable sourceUrl so relative manifest artifacts can be resolved from the manifest URL.

Testing instructions

npx nx typecheck playground-blueprints
npx nx build:blueprint-schema playground-blueprints
cd packages/playground/blueprints
npx vitest run src/lib/steps/load-php-extension.spec.ts --config vite.config.ts
npx vitest run src/lib/v1/resources.spec.ts --config vite.config.ts -t "recognizes literal directory resources"