Refine Columbia sync after first live run

Fixes from reviewing the first full sync of the real catalog:

- Exclude non-powder / size-variant listings: physical swatch cards (-SW /
  "SWATCH"), 4 oz testers (-04 / "Tester"), and 5 lb sample bags ("Sample (").
  These are not standalone powder colors. Filtered before mapping, and a
  cleanup step deletes any already synced (so they're removed, not flagged
  discontinued). Sample detection keys off the "Sample (" name, not the bare
  -S suffix, to avoid catching a real SKU ending in S (verified 0 collisions).
- Tighten RequiresClearCoat: was flagging ~53% of the catalog on any casual
  "clear coat" mention. Now only genuine signals (partial-cure schedules, the
  Illusion line, explicit "requires a clear" phrasing) trip it.
- Fix literal "—" in the sync success banner (TempData is HTML-encoded).

Tests cover the exclusion patterns and the tightened clear-coat detection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/PowderCoating.Web/Controllers/PowderCatalogController.cs

@@ -445,7 +445,7 @@ public class PowderCatalogController : Controller
         var result = await _columbiaSyncService.RunSyncAsync(cancellationToken);
 
         if (result.Success)
-            TempData["Success"] = $"Columbia sync complete — {result.Summary}";
+            TempData["Success"] = $"Columbia sync complete - {result.Summary}";
         else
             TempData["Error"] = $"Columbia sync failed: {result.ErrorMessage}";