Preserve post-runtime blueprint steps across cache hits

[chubes4]

Summary

Preserve caller-specific post-runtime blueprint steps while retaining reusable prepared-runtime cache state.

What changed

• Cache only shared runtime steps, append each caller's post-runtime tail after cache selection, and persist a session-specific hydratable blueprint ref.

How to test

1. Run npm run test:php-browser-preview-only-session; expect The PHP smoke passes and proves miss/hit hydration preserves each caller import without cross-session leakage..
2. Run TMPDIR=/tmp npm run smoke -- --group package; expect The package build smoke passes..

Compatibility

No public schema removal. Existing sessions without post_runtime_blueprint keep their current prepared-runtime refs; caller-specific sessions now hydrate the complete executable blueprint.

Evidence

• Reviewer-resolvable source evidence: Preserve post-runtime blueprint steps across prepared runtime cache hits #1812
• package-smoke: Passed
• preview-session-regression: Passed

AI assistance

• AI assistance: Yes
• Tool(s): Homeboy and OpenCode
• Model: openai/gpt-5.6-terra
• Used for: Diagnosed the prepared-runtime cache interaction, drafted the implementation and regression coverage, and orchestrated verification; Chris reviews and owns the change.

Source relationships

• Closes Preserve post-runtime blueprint steps across prepared runtime cache hits #1812