feat: deterministic image composer - OG + square variants, branding, baked caption (v0.17.0) (#152)

Composer slots between provider bytes and sideload: corner-marked featured,
branded 1200x630 OG + 1080x1080 square with baked clamped caption, capability
ladder (Imagick -> resize-only -> pass-through, never blocks publish).
Alt-text bug fix (caption, not model name). Vendored brand PNGs + OFL Poppins.

QA: APPROVE-WITH-P2-NOTES @ 6f76df1 (qa-reviews/prautoblogger/2026-06-11-6f76df1.md)
CEO greenlight: 2026-06-11 in chat.

Agent-Session: cto-image-composer-merge-20260611

includes/core/interface-image-composer.php

@@ -0,0 +1,50 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * phpcs:ignore WordPress.Files.FileName.InvalidClassFileName -- class naming convention differs from WordPress standard
+ *
+ * Contract for image composers that turn raw provider bytes into final variants.
+ *
+ * A composer sits between the image provider (raw generated bytes) and the
+ * media sideloader. It receives one base image plus per-post context and
+ * returns one or more finished variants — the first is always the `featured`
+ * role (the bytes that become the post thumbnail), optionally followed by
+ * social variants (`og`, `square`). Implementations must NEVER throw out of
+ * compose(): on any internal failure they degrade to passing the base image
+ * through unmodified so publishing is never blocked.
+ *
+ * Triggered by: PRAutoBlogger_Image_Pipeline between batch results and sideload.
+ * Dependencies: none — this is the contract only.
+ *
+ * @see core/class-image-composer.php  — Default implementation (capability ladder).
+ * @see core/class-image-pipeline.php  — Caller.
+ * @see core/class-image-attacher.php  — Persists the returned variants.
+ */
+interface PRAutoBlogger_Image_Composer_Interface {
+
+	/**
+	 * Compose final image variant(s) from raw provider bytes.
+	 *
+	 * Side effects: implementations may read plugin options and write to the
+	 * activity log; they must not make network calls or fail the publish.
+	 *
+	 * @param array{
+	 *     bytes: string,
+	 *     mime_type: string,
+	 *     width: int,
+	 *     height: int,
+	 * } $image_data Base image bytes + metadata from the provider result.
+	 * @param array{
+	 *     post_id: int,
+	 *     caption: string,
+	 *     title: string,
+	 *     slot: string,
+	 * } $context Post ID, editorial caption, post title, and slot
+	 *            ('image_a' or 'image_b'); only image_a yields variants.
+	 *
+	 * @return array<int, array{bytes: string, mime_type: string, width: int, height: int, role: string}>
+	 *               Non-empty list; index 0 always carries role 'featured'.
+	 */
+	public function compose( array $image_data, array $context ): array;
+}