gh-40171: much faster count for linear intervals
    
by caching the partial order in the adequate way

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40171
Reported by: Frédéric Chapoton
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/combinat/posets/posets.py

@@ -2772,6 +2772,10 @@ def linear_intervals_count(self) -> list[int]:
         """
         if not self.cardinality():
             return []
+        # precomputation helps for speed:
+        if self.cardinality() > 60:
+            self.lequal_matrix()
+
         H = self._hasse_diagram
         stock = [(x, x, x) for x in H]
         poly = [len(stock)]