fix(registry): clear brand CSS custom properties on unbranded re-attach; remove dead italic path

hfApplyStageConfig (Block B, verbatim across all 5 retrofitted caption
identities) only ever set --hf-caption-primary/--hf-caption-accent when the
corresponding brand.primaryColor/accentColor key was present, with no else
branch to clear it when absent. Re-attaching a brand-less payload after a
branded one left the custom property (and any JS-cached color derived from
it, e.g. caption-pill-karaoke's hfColorActive and caption-emoji-pop's
hfAccentColors) stuck at the stale value instead of reverting to the CSS
fallback, violating idempotent re-attach.

Also removes caption-editorial-emphasis's dead .word--italic CSS rule and
CLASS_MAP.i entry — hfMakeBlocks only ever emits "n"/"e" tags, "i" was only
reachable via the old hand-authored BLOCKS literal this task replaced.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

registry/components/caption-editorial-emphasis/caption-editorial-emphasis.html

@@ -95,13 +95,6 @@
         font-style: normal;
       }
 
-      .word--italic {
-        font-family: "Inter", sans-serif;
-        font-size: 86px;
-        font-weight: 400;
-        font-style: italic;
-      }
-
       .word--emphasis {
         font-family: "Playfair Display", serif;
         font-size: 180px;
@@ -236,9 +229,13 @@
         var brand = (data && data.brand) || {};
         if (typeof brand.primaryColor === "string" && brand.primaryColor) {
           root.style.setProperty("--hf-caption-primary", brand.primaryColor);
+        } else {
+          root.style.removeProperty("--hf-caption-primary");
         }
         if (typeof brand.accentColor === "string" && brand.accentColor) {
           root.style.setProperty("--hf-caption-accent", brand.accentColor);
+        } else {
+          root.style.removeProperty("--hf-caption-accent");
         }
         return { W: W, H: H, scaleX: W / 1920, scaleY: H / 1080, fontScale: Math.min(W, H) / 1080 };
       }
@@ -415,7 +412,7 @@
         return minSize;
       }
 
-      var CLASS_MAP = { n: "word word--normal", i: "word word--italic", e: "word word--emphasis" };
+      var CLASS_MAP = { n: "word word--normal", e: "word word--emphasis" };
 
       var W = [];
       var BLOCKS = [];