Fix world briefing & add LLM enrichment, colophon fix, x4 cover fix

- World Briefing now starts with yesterday, preserves complete nested
event hierarchies and Wikipedia links, fetches deduplicated article
leads concurrently, and adds stable-ID summaries plus a daily overview.
- Enrichment runs after normal editorial work and degrades safely with
report warnings.
- Colophon facts now use X4-safe block paragraphs.
- Standard cover remains 1200×1600 RGB PNG.
- X4 cover is a 480×800 baseline RGB JPEG with consistent XHTML and OPF
declarations.
- Added CrossInk cache-clearing guidance to README.md.
- Worked around an epub-builder duplicate XML-ID defect discovered by
validation.

src/epub/templates/style.css

@@ -275,16 +275,20 @@ blockquote.comment blockquote.comment {
   margin-bottom: 0.25em;
 }
 
+.world-summary {
+  margin: 0.2em 0 0.45em 0;
+  font-style: italic;
+}
+
+.world-overview {
+  margin-bottom: 0.7em;
+}
+
 .attribution {
   font-size: 0.8em;
   font-style: italic;
 }
 
-.colophon-facts dt {
-  font-variant: small-caps;
-  margin-top: 0.4em;
-}
-
-.colophon-facts dd {
-  margin: 0 0 0 1em;
+.fact-line {
+  margin: 0 0 0.35em 0;
 }