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>

CONVENTIONS.md

@@ -312,11 +312,14 @@ psa_get_admin_user_id()             /* Get first admin user ID */
 
 ```php
 PSA_Config                          /* Configuration constants */
-PSA_Error                           /* Error handling [deprecated] */
 PSA_Post_Type                       /* CPT registration & meta */
 PSA_Search                          /* Search, AJAX, REST API, shortcode */
-PSA_AI_Generator                    /* OpenRouter integration */
+PSA_AI_Generator                    /* AI pipeline orchestrator */
+PSA_OpenRouter                      /* OpenRouter API communication layer */
+PSA_KB_Builder                      /* Echo Knowledge Base article builder */
 PSA_PubChem                         /* PubChem API integration */
+PSA_Cost_Tracker                    /* API usage tracking & budget enforcement */
+PSA_Encryption                      /* AES-256-CBC API key encryption */
 PSA_Admin                           /* Settings pages & fields */
 PSA_Template                        /* Single page rendering */
 ```