Fix post-runtime blueprint ref hydration

[chubes4]

Summary

Fixes #1812.

• Keep the prepared-runtime artifact limited to shared runtime materialization.
• Persist the complete executable blueprint behind a session-bound hydration ref only when a caller supplied post_runtime_blueprint.
• Cover a cache miss followed by a hit with two distinct SSI import tails, asserting each hydrated DTO has the current tail after shared steps and never the other tail.

Evidence

• Source relationship: post_runtime_blueprint is intentionally excluded from prepared-runtime hashing in browser_prepared_runtime_contract; it must remain outside reusable cache identity.
• Change kind: targeted runtime cache/hydration-ref persistence fix plus deterministic PHP smoke regression.
• Verification capability: the smoke uses in-memory WordPress transient stubs, so it deterministically exercises miss, shared hit, hydration, ordering, and cross-session isolation without external services.
• Scope: runtime behavior changes only when a non-empty caller tail is present. Shared artifacts still use the existing cache key and hash; tail-bearing blueprints are stored under a hash containing the session and executable blueprint, so they cannot become reusable cache state or leak to another session.

Testing

1. Run npm run test:php-browser-preview-only-session.
2. Run php scripts/php-browser-post-runtime-blueprint-smoke.php.
3. Optionally run php -l packages/wordpress-plugin/src/class-wp-codebox-browser-task-builder.php && php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-execution.php && php -l scripts/php-browser-preview-only-session-smoke.php.

Compatibility

The public prepared:<cache_key>:<hash> ref format and hydrator contract are unchanged. Sessions without post_runtime_blueprint retain existing shared-cache behavior. Sessions with a caller tail receive a distinct, hydratable ref for their executable blueprint.

AI Disclosure

Implemented with OpenAI Codex assistance; changes and test results were reviewed in the repository.