Commit 1: editorial image prompt + style-template setting

Pivot the image pipeline from the single-panel comic style to a text-free
editorial scientific illustration, and replace the Style Suffix setting with
an editable Style Template setting. Commit 1 of the in-plugin editorial image
pipeline (no composer yet; pipeline otherwise unchanged).

- New constant PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_TEMPLATE (brief §7 editorial
  template with the single {{ topic_summary }} token).
- New setting prautoblogger_image_style_template (textarea) replaces the
  Style Suffix field; single-token validation on save (brief A5); old
  prautoblogger_image_style_suffix mirrored to _deprecated for one cycle and
  not deleted (one-time migration prautoblogger_migrated_style_template_v0160).
- REWRITER_SYSTEM_PROMPT now emits a 1-2 sentence editorial topic/mechanism
  summary as the SCENE (no comic gag), keeping the CAPTION line for the
  unchanged HTML-caption-below-image path; scene/caption parsing contract
  preserved.
- New PRAutoBlogger_Image_Template_Filler substitutes the scene into the
  template's {{ topic_summary }} slot across all three entry points; strips
  control chars + clamps length (A5); degrades safely on missing token or
  empty summary so a blank/token-only prompt is never sent to the provider
  (A5/A6).
- Version bump 0.15.1 -> 0.16.0 + CHANGELOG + ARCHITECTURE.md updates.

Agent-Session: image-editorial-commit1

includes/core/class-image-scene-parser.php

@@ -70,22 +70,25 @@ class PRAutoBlogger_Image_Scene_Parser {
 	/**
 	 * Rule-based fallback when LLM rewriting is unavailable.
 	 *
-	 * Kept deliberately simple — this only runs if OpenRouter is down.
+	 * Kept deliberately simple — this only runs if OpenRouter is down. Produces
+	 * a text-free editorial-illustration subject (not a comic) so the prompt
+	 * builder always has a non-empty topic summary to substitute into the
+	 * style template, never shipping a blank prompt to the provider (brief A6).
 	 *
 	 * @param string $title           Main heading / topic.
 	 * @param string $supporting_text Additional context.
 	 * @return array{scene: string, caption: string} Scene for image gen, caption for HTML.
 	 */
 	public static function synthesize_visual_concepts_fallback( string $title, string $supporting_text ): array {
-		// Fallback produces a simple comic concept when the LLM is unavailable.
+		// Fallback produces a simple editorial-illustration subject when the LLM is unavailable.
 		$scene = sprintf(
-			'A cartoon scientist in a lab coat looking bewildered while examining something related to: %s.',
+			'A clean, centered editorial science illustration representing the topic of %s, with a single clear focal subject and generous negative space.',
 			$title
 		);
 
 		return array(
 			'scene'   => trim( $scene ),
-			'caption' => 'Science is full of surprises.',
+			'caption' => '',
 		);
 	}
 }