repertoire: HTMX OOB swaps for filter columns + fix fading edge cases

Switch from swapping the entire #repertoire-index block to targeted
hx-swap-oob swaps: only the <ul> list elements are replaced, while
section headers and accordion chrome stay in the DOM untouched.

- Filter column <ul>s get IDs (rep-list-years, rep-list-ap, etc.)
  and render with hx-swap-oob=true on HTMX requests
- Students <ul> gets id=rep-students as main swap target
- Filter buttons now target #rep-students instead of #repertoire-index
- Server sets $isOob=true when rendering HTMX partial responses
- Accordion re-init on swap no longer needed (headers never replaced)
- Scroll-restore updated to capture/restore per <ul> ID

Fading fixes:
- Restore rep-entry--faded class with simplified logic: any active
  filter fades entries whose matched flag is false (removed the
  $colHasMatch gate that prevented fading when a column had no matches)
- Add $noResults guard: when matched_ids is empty (zero theses match
  all active filters), force-fade all non-selected entries across ALL
  columns. This covers the keyword edge case where per-column matched
  is computed excluding the keyword filter (many-to-many), causing
  keywords to appear valid even though the full intersection is empty.

app/public/assets/css/repertoire.css

@@ -359,6 +359,13 @@
   color: var(--accent-primary);
 }
 
+/* Faded/disabled: no valid results for this filter combination */
+.rep-entry--faded {
+  opacity: 0.3;
+  cursor: not-allowed;
+  pointer-events: none;
+}
+
 /* Years column — big numbers, semi-bold (BBBDMSans Medium weight) */
 .repertoire-col[data-col="years"] .rep-entry {
   font-size: var(--step-3);