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P1 fix: Add class_exists() guard in PRC_Preset_Provider before instantiating PR Core repository classes. If PR Core constant is defined but classes fail to autoload, fall back to defaults instead of crashing. P2 fixes: - Remove unused nonce from wp_localize_script (all endpoints public) - Add cross-reference comments pinning SYRINGE_UNITS_PER_ML across PRC_Math, calculator.js, and prcConfig.syringeUnits QA verdict: qa-reviews/peptide-reconstitution-calculator/2026-04-17-9aa5309.md Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>