Preserve dependency overlay source references on Lab snapshots

[chubes4]

Problem

A clean Git-backed composer-package dependency overlay loses its source reference in a Lab fixture-matrix recipe, even after the package layout and structured WP-CLI output paths are repaired.

Evidence

• Lab run: 50eadcfc-23c9-450a-929f-cb47f08f2a65
• Combined WP Codebox integration commit: 427cd1514bd157cc4fcf07cf6744acb1eb7d7d98 (Preserve structured WP-CLI recipe results #2054 + Preserve wrapped Composer overlay package layouts #2057)
• Blocks Engine source commit: 9ebb3a71790afd3068dedae79340088208e11c81
• The controller and independently materialized Lab snapshots both report clean Git status.
• Structured output now parses and retains the import report, WordPress site plan, completed materialization receipt, and eight page provenance records.
• Remaining evidence state: legacy_evidence_missing, missing only transformer_reference.
• Runtime transformer identity: package automattic/blocks-engine-php-transformer, version 1.0.0+no-version-set, empty reference.

Expected

prepareRecipeDependencyOverlays() should preserve the clean overlay commit in the consumer Composer metadata read by Composer\InstalledVersions::getReference(), including wrapper-layout staging and Lab snapshot materialization. Add deterministic coverage using an actual clean Git source instead of a non-Git temporary package.