Support custom WordPress roots and full browser-action arguments on the native backend

Follow-up to #2473 / #2475.

## Problem

The `wordpress-native` backend runs stock WordPress from its pinned image and implements only part of the browser-action contract. Downstream consumers that supply their own WordPress distribution would silently execute against stock WordPress instead of their source, which produces misleading evidence.

## Gaps

1. `runtime.assets.wordpressDirectory` is resolved by the CLI (`packages/cli/src/commands/recipe-run.ts`) and honored by Playground, but the native driver ignores it.
2. `wordpress.browser-actions` on native ignores `route-host`, `auth-user-id`, `capture`, `step-timeout`, and `timeout`.

## Acceptance criteria

- A backend-neutral recipe supplying a custom WordPress root executes that source on the native backend, with provenance identifying the mounted source rather than the image default.
- Selecting native versus Playground requires no workflow command changes.
- `route-host` maps declared hosts to the contained runtime so host-dependent routing behaves as it does on Playground.
- `auth-user-id` authenticates as the requested fixture user; only runtime-generated fixture credentials are used.
- `capture` honors its declared streams, and unsupported tokens fail explicitly instead of being ignored.
- `step-timeout` and `timeout` bound individual steps and the whole action, and timeouts surface as typed, bounded failures with retained evidence.
- Unsupported arguments fail closed with actionable errors rather than silently degrading.
- Deterministic coverage plus a containment-gated live test proving a custom WordPress root and the full argument set.

## Non-goals

- Consumer-specific paths, fixtures, or product names in runtime-core or the native package.

## AI assistance

AI assistance: OpenCode with openai/gpt-5.6-sol was used to verify the gaps in the CLI and native driver and draft this issue. Chris reviewed and owns the report.

Start with packages/cli/src/commands/recipe-run.ts and compare its custom WordPress root handling with the native driver and Playground behavior. Trace the native browser-action path for route-host, auth-user-id, capture, step-timeout, and timeout, then run the deterministic coverage and containment-gated live test. Done means custom roots and all declared arguments work on native, while unsupported arguments fail explicitly.

Written by the indexing model from the issue text.