Persist typed browser probe results for downstream recipe steps

## Problem
Recipe browser probes can evaluate runtime page state, but cannot write the typed result to a caller-selected artifact path that a later recipe step can consume. This blocks pre-compilation runtime presentation evidence workflows.

## Direction
Add a generic output-artifact contract to browser probe/page-evaluation commands. Persist the typed result atomically within the declared artifact root, register an artifact ref, and make it available to subsequent workflow steps.

## Acceptance
- Caller can declare a safe relative output artifact path.
- Typed browser result is atomically persisted and registered.
- A later recipe step can read the file in the same run.
- Path traversal and writes outside the artifact root are rejected.
- Failure evidence distinguishes evaluation, serialization, and persistence errors.
- Existing browser probe behavior is unchanged when omitted.
- Contract and recipe integration tests cover success and rejection.

## Dependency
Automattic/static-site-importer#796, producer commit `f246198c`; Automattic/blocks-engine#757.

## AI assistance
OpenAI gpt-5.6-sol through OpenCode analyzed the cross-recipe gap and drafted this issue. Chris Huber remains responsible for scope and implementation decisions.

Start by locating the browser probe/page-evaluation commands and the recipe artifact registration flow. Trace how a typed result moves between recipe steps, then add contract and recipe integration tests covering successful reads and rejected paths; done means omitted output preserves existing behavior and persistence failures are distinguished.

Written by the indexing model from the issue text.