feat: scroll-position memory on repertoire HTMX swaps + swap transition polish

- Add repertoire-scroll-restore.js: snapshots scrollTop of each column <ul>
  before htmx:beforeSwap, restores after htmx:afterSwap (keyed by data-col)
- Add subtle opacity transition on #repertoire-index during htmx-swapping
- Tighten rep-indicator opacity transition to 0.1s for snappier feedback
- Import new module in public-entry.js

app/public/assets/css/repertoire.css

@@ -290,6 +290,17 @@
     color: var(--accent-primary);
 }
 
+/* ── Swap transition ────────────────────────────────────────────────── */
+
+.repertoire-index.htmx-swapping {
+    opacity: 0.5;
+    transition: opacity 0.1s ease-out;
+}
+
+.repertoire-index {
+    transition: opacity 0.15s ease-in;
+}
+
 /* Link variant (students col) — no underline by default */
 .rep-entry--link {
     text-decoration: none;
@@ -333,7 +344,7 @@
     height: 2px;
     background: var(--accent-primary);
     opacity: 0;
-    transition: opacity 0.15s;
+    transition: opacity 0.1s;
     z-index: 100;
     pointer-events: none;
 }