fix(schema): v0.6.2 — move Drug/FAQ injection to wpseo_schema_graph; fix peptide 500

Root cause: inject_graph_pieces() added plain PHP arrays to wpseo_schema_graph_pieces.
Yoast Schema_Generator::filter_graph_pieces_to_generate() calls get_class() +
is_needed() on every piece expecting Abstract_Schema_Piece objects — a plain array
produces: PHP Fatal: get_class(): Argument #1 ($object) must be of type object,
array given. All peptide monograph pages returned HTTP 500; non-peptide pages were
unaffected (is_singular guard).

Fix: remove wpseo_schema_graph_pieces hook entirely. New inject_graph_nodes() method
hooks wpseo_schema_graph at priority 12 (after enrich_webpage_piece at 11), which
receives the fully-assembled graph as array-of-arrays — safe for plain array appends.

Also adds regression test (test-jsonld-peptide-piece.php) that:
- Reproduces the v0.6.1 get_class(array) TypeError (fails-before)
- Confirms inject_graph_nodes Drug+FAQ emission is correct (passes-after)

Agent-Session: jsonld-peptide-piece-fix v0.6.2

peptide-repo-core.php

@@ -3,7 +3,7 @@
  * Plugin Name: Peptide Repo Core
  * Plugin URI:  https://peptiderepo.com
  * Description: Canonical peptide schema — shared data layer for the peptiderepo.com ecosystem. Provides the peptide CPT, dosing rows, legal status cells, AI candidate queue, disclaimer component, and JSON-LD output.
- * Version:     0.6.1
+ * Version:     0.6.2
  * Author:      peptiderepo
  * Author URI:  https://peptiderepo.com
  * License:     GPL-2.0-or-later
@@ -22,7 +22,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 
 /* ── Constants ────────────────────────────────────────────────────────── */
 
-define( 'PR_CORE_VERSION', '0.6.1' );
+define( 'PR_CORE_VERSION', '0.6.2' );
 define( 'PR_CORE_PLUGIN_FILE', __FILE__ );
 define( 'PR_CORE_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
 define( 'PR_CORE_PLUGIN_URL', plugin_dir_url( __FILE__ ) );