recipe runtime.backend: "wordpress" is accepted by the schema, then silently runs wordpress-playground

The workspace recipe schema advertises two runtimes:

```json
"backend": { "enum": ["wordpress-playground", "wordpress"] }
```

A recipe declaring `"backend": "wordpress"` **validates clean**, and then runs on Playground anyway:

```
$ wp-codebox recipe validate --recipe ./native.json --json
"valid": true

$ wp-codebox recipe-run --recipe ./native.json
WP Codebox recipe
Runtime: wordpress-playground <-- not what the recipe declared
```

Minimal recipe that reproduces it:

```json
{
"schema": "wp-codebox/workspace-recipe/v1",
"runtime": { "backend": "wordpress", "name": "native-probe", "wp": "7.0", "blueprint": { "steps": [] } },
"workflow": { "steps": [ { "command": "wordpress.run-php", "args": ["code=echo \"NATIVE_OK\";"] } ] },
"artifacts": { "directory": "artifacts/native-probe" }
}
```

Shipped packages are `cli`, `runtime-core`, `runtime-playground`, `runtime-cloudflare` — there is no native runtime, so the fallback looks intentional. The problem is that it is silent.

## Why this is worth fixing

A declared runtime that quietly becomes a different runtime is hard to reason about from the outside. I spent a long stretch attributing failures to the Playground/WASM runtime while believing I had opted out of it, because both validation and the run reported success paths that never mentioned the substitution.

## Suggested options

1. Reject `"backend": "wordpress"` at validation time while no native runtime is installed, with a blocker naming the available backends, or
2. Keep accepting it and emit a structured observation on the run ("declared backend `wordpress` resolved to `wordpress-playground`"), or
3. Drop the value from the enum until a native runtime ships.

Any of the three is better than a silent substitution.

---

AI assistance: GPT-6 Astra via OpenCode found this while trying to run a large WordPress source tree outside WASM, and confirmed the fallback with the minimal recipe above.

Start by reproducing the behavior with the supplied native.json using `wp-codebox recipe validate` and `recipe-run`. Trace how the recipe schema accepts `wordpress` and how the shipped runtime packages resolve it, then determine the project's preferred handling for an unavailable backend. Done means validation or execution clearly prevents or reports the substitution, with coverage for the minimal recipe.

Written by the indexing model from the issue text.