feat: add browsable peptide directory with category filtering, REST API, and extended meta fields (v4.3.0)

Adds [peptide_directory] shortcode with card grid, category taxonomy (8 terms),
search, pagination, detail modal, and /v1/compounds REST endpoint. Includes 9
new meta fields (half-life, stability, solubility, etc.), admin data tools for
migration/re-enrichment, integration hooks for cross-plugin extensibility,
dark mode support, and full uninstall cleanup. Refactors AI generator into
PSA_OpenRouter + PSA_KB_Builder classes. Adds PHPUnit tests for PostType and
Directory classes. Fixes QA findings: dead JS selector, renderError loading
state, numeric meta validation, prepared SQL in uninstall.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/class-psa-config.php

@@ -1,7 +1,11 @@
 <?php
+declare( strict_types=1 );
 /**
  * Configuration constants for Peptide Search AI.
- * Centralizes all magic numbers and configuration values.
+ *
+ * What: Centralizes all magic numbers and configuration values used across the plugin.
+ * Who calls it: Every class in the plugin reads these constants for limits, timeouts, and defaults.
+ * Dependencies: None (pure constants class).
  *
  * @package PeptideSearchAI
  */