fix(blog): make .is-hidden global, strip HTML tags from search text, add role=search

Final-review findings:

- .is-hidden was scoped to blog/index.astro, so it didn't match PostRow
  or YearHead roots (which carry their own component scope hashes). Net
  effect: the filter visibly only hid the featured article, while all
  archive rows + year heads stayed on screen. Wrap in :global() so the
  runtime-toggled class actually fires its display rule everywhere.
- buildSearchText regex stripped > but not <, so inline HTML open-tags
  like <span class="leadin"> leaked into data-search. Add < to the
  character class so the haystack stays plain text.
- Add role="search" landmark to the BlogSearch wrapper so screen-reader
  users can navigate to it directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/blog/index.astro

@@ -43,7 +43,7 @@ function buildSearchText(post: (typeof posts)[number]): string {
   return parts
     .join(" ")
     .toLowerCase()
-    .replace(/[#*_>`\[\]()]/g, " ")
+    .replace(/[<>#*_`\[\]()]/g, " ")
     .replace(/\s+/g, " ")
     .trim();
 }
@@ -265,7 +265,9 @@ for (const post of posts) {
     .feature .img { order: -1; aspect-ratio: 16 / 9; }
   }
 
-  .is-hidden { display: none !important; }
+  /* :global so the runtime-toggled class reaches PostRow and YearHead roots,
+     which carry their own component scope hashes — not this page's. */
+  :global(.is-hidden) { display: none !important; }
 
   .empty {
     font-family: var(--font-mono);