feat: ship Peptide Reconstitution Calculator v1.0.0

Interactive reconstitution calculator with peptide-specific presets.

Core features:
- [prc_calculator] shortcode with instant client-side math
- Concentration (mg/mL), mcg/unit, injection volume, syringe units, doses/vial
- Live presets from PR Core dosing data (graceful fallback to 8 defaults)
- REST API (prc/v1): presets list/detail + server-side calculate
- Transient-cached presets with event-driven invalidation
- Dark mode + responsive design (320px+)
- Full teardown in uninstall.php

12 PHP files (max 241 lines), 1 JS file, 1 CSS file.
CI: PHP lint 8.1-8.3, PHPCS WordPress, 300-line check.

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

includes/class-prc-autoloader.php

@@ -0,0 +1,67 @@
+<?php
+/**
+ * SPL autoloader for PRC_-prefixed classes.
+ *
+ * What: Maps PRC_ class names to kebab-case filenames under includes/.
+ * Who calls it: Required once in the bootstrap file (peptide-reconstitution-calculator.php).
+ * Dependencies: None.
+ *
+ * @see class-pr-core-autoloader.php in Peptide Repo Core — same pattern.
+ */
+
+declare(strict_types=1);
+
+/**
+ * Autoloads PRC_ classes from includes/ and its immediate subdirectories.
+ */
+class PRC_Autoloader {
+
+	/**
+	 * Register the autoloader with SPL.
+	 *
+	 * @return void
+	 */
+	public static function register(): void {
+		spl_autoload_register( [ __CLASS__, 'autoload' ] );
+	}
+
+	/**
+	 * Autoload callback — resolve PRC_ class name to file path.
+	 *
+	 * Strips PRC_ prefix, converts underscores to hyphens, lowercases,
+	 * prepends "class-prc-", and searches includes/ + subdirs.
+	 *
+	 * @param string $class_name Fully qualified class name.
+	 * @return void
+	 */
+	public static function autoload( string $class_name ): void {
+		if ( 0 !== strpos( $class_name, 'PRC_' ) ) {
+			return;
+		}
+
+		$relative = substr( $class_name, 4 ); // Strip "PRC_".
+		$filename = 'class-prc-' . str_replace( '_', '-', strtolower( $relative ) ) . '.php';
+
+		$base = PRC_PLUGIN_DIR . 'includes/';
+
+		// Check includes/ root first.
+		if ( file_exists( $base . $filename ) ) {
+			require_once $base . $filename;
+			return;
+		}
+
+		// Check immediate subdirectories.
+		$dirs = glob( $base . '*', GLOB_ONLYDIR );
+		if ( ! is_array( $dirs ) ) {
+			return;
+		}
+
+		foreach ( $dirs as $dir ) {
+			$path = $dir . '/' . $filename;
+			if ( file_exists( $path ) ) {
+				require_once $path;
+				return;
+			}
+		}
+	}
+}