feat(schema): v0.6.0 — PSA backfill migration + Yoast-integrated Drug/MedicalWebPage/FAQ schema

Piece 1 — Backfill migration 0004: copies PSA psa_molecular_formula/weight/aliases
into PR Core _pr_* meta. Idempotent + re-runnable. PubChem REST fallback for 4
posts missing PSA data (MAX_RETRIES=3, exp backoff). All PSA values UNTRUSTED.
PR_CORE_TARGET_SCHEMA_VERSION bumped 3 -> 4.

Piece 2 — Drug node enrichment + stable @id = {permalink}#drug (ratified contract).
Reads _pr_* schema-input meta. molecularWeight in g/mol. Empty-prop guard throughout.

Piece 3 — Yoast-integrated emission via wpseo_schema_graph_pieces (priority 11),
wpseo_schema_webpage_type (MedicalWebPage retype), wpseo_schema_graph (enrichment).
Standalone @graph fallback retained. No duplicate WebPage/BreadcrumbList.
FAQPage emitted only when _pr_faq_items populated.

Supporting: PR_Core_Schema_Sanitizers, PR_Core_Pubchem_Client, CONTEXT.md,
ARCHITECTURE.md, CONVENTIONS.md, CHANGELOG.md, uninstall.php purge.
Tests: test-migration-0004.php (20 assertions), test-jsonld.php (29 assertions).

Agent-Session: peptide-schema-backfill v0.6.0

uninstall.php

@@ -76,6 +76,31 @@ $wpdb->query(
 	"DELETE FROM {$wpdb->options} WHERE option_name LIKE 'pr\_core\_%'"
 );
 
+/* ── 4b. Delete v0.6.0 schema-input meta keys from all peptide posts ──── */
+
+/*
+ * Migration 0004 writes _pr_molecular_formula, _pr_molecular_weight, _pr_aliases,
+ * and _pr_faq_items to the shared peptide posts. These are PR Core-authored values
+ * (derived from PubChem / PSA) and are safe to remove on uninstall.
+ *
+ * @see ARCHITECTURE.md — §2.9 Uninstall specification.
+ */
+$schema_meta_keys = [
+	'_pr_molecular_formula',
+	'_pr_molecular_weight',
+	'_pr_aliases',
+	'_pr_faq_items',
+];
+
+foreach ( $schema_meta_keys as $meta_key ) {
+	// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery
+	$wpdb->delete(
+		$wpdb->postmeta,
+		[ 'meta_key' => $meta_key ],
+		[ '%s' ]
+	);
+}
+
 /* ── 5. Remove manage_peptide_content capability from all roles ───────── */
 
 $editable_roles = wp_roles()->roles;