feat(credits): restore the CC BY-SA declaration as "License & reuse" (Option D1)

s0031 removed "About this work" to avoid duplicating it on the separate About
page, but that section WAS the D1 compliance — it declares the altered work is a
derivative offered under CC BY-SA 4.0, which is what satisfies the ShareAlike
obligation for the BY-SA source clips. Without it the live site credited the
BY-SA authors but never declared the derivative share-alike.

Restore the declaration on the credits page, under the heading "License & reuse"
instead of "About this work" so it isn't confused with the separate About page
(which covers the project's intent). Tests assert the declaration is present and
that the heading is NOT "About this work".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/static/credits.html

@@ -13,12 +13,30 @@
       <p><a href="index.html" class="back-link">← Back to the experience</a></p>
     </header>
 
+    <!-- Option D1: the altered work's own license + the "modified" statement that
+         CC BY / CC BY-SA attribution requires. Heading deliberately NOT "About" — the
+         separate About page covers the project's intent. -->
+    <section class="declaration">
+      <h2>License &amp; reuse</h2>
+      <p>
+        The <em>Human Experience Filter</em> alters its source footage in real time
+        (overlays, an emotion/affect channel, and a WebGL “dream” shader), so every
+        frame you see is a <strong>derivative</strong> of the original clip.
+      </p>
+      <p>
+        This altered work is offered under
+        <a href="https://creativecommons.org/licenses/by-sa/4.0/" rel="license">Creative
+        Commons Attribution-ShareAlike 4.0 (CC BY-SA 4.0)</a>. The original sources it
+        builds on are credited below. This is a non-commercial art installation.
+      </p>
+    </section>
+
     <section>
       <h2>Footage</h2>
       <p class="note">
         Original video sources and their licenses. Public-domain (US-government) and
         CC0 clips need no attribution; the names below for CC BY / CC BY-SA clips are
-        required credit. All footage is altered in real time in the experience.
+        required credit. All footage is altered as described above.
       </p>
       <div id="video-credits" class="credits-list">
         <p class="loading">Loading credits…</p>