V3Covergroup: fix coverpoint cleanup for unsupported covergroups

Two bugs fixed:
1. AstCReset: mark as ineligible for coverage expressions via
   isExprCoverageEligible() override, preventing verilogForTree
   from being called on CReset nodes (which has no V3EmitV handler).

2. generateCrossCode: when a cross references an unknown coverpoint,
   don't delete the cross node early. The caller's cleanup loop
   (in visit(AstClass*)) is responsible for deleting all coverpoints
   and crosses. Early deletion left a dangling pointer in m_coverCrosses
   causing a use-after-free segfault.

3. hasUnsupportedEvent path: added coverpoint/cross cleanup before
   returning so AST nodes don't reach downstream passes (V3EmitCFunc,
   V3MergeCond) which no longer have stub visitors for them.

All 60 covergroup tests now pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/V3AstNodeExpr.h

@@ -690,6 +690,7 @@ public:
     string emitVerilog() override { V3ERROR_NA_RETURN(""); }
     string emitC() override { V3ERROR_NA_RETURN(""); }
     bool cleanOut() const override { return true; }
+    bool isExprCoverageEligible() const override { return false; }
     const char* broken() const override {
         BROKEN_RTN(!VN_IS(backp(), NodeAssign));  // V3Emit* assumption
         return nullptr;

src/V3Covergroup.cpp

@@ -1345,8 +1345,7 @@ class FunctionalCoverageVisitor final : public VNVisitor {
                     refp->v3warn(COVERIGN,
                                  "Ignoring unsupported: cross references unknown coverpoint: "
                                      + refp->name());
-                    // Delete the entire cross since we can't generate it
-                    VL_DO_DANGLING(crossp->unlinkFrBack()->deleteTree(), crossp);
+                    // Don't delete crossp here - the caller's cleanup loop will delete it
                     return;
                 }
 
@@ -1830,7 +1829,19 @@ class FunctionalCoverageVisitor final : public VNVisitor {
             }
 
             // If covergroup has unsupported clocking event, skip processing it
-            if (hasUnsupportedEvent) return;
+            // but still clean up coverpoints so they don't reach downstream passes
+            if (hasUnsupportedEvent) {
+                iterateChildren(nodep);
+                for (AstCoverpoint* cpp : m_coverpoints) {
+                    cpp->unlinkFrBack();
+                    VL_DO_DANGLING(cpp->deleteTree(), cpp);
+                }
+                for (AstCoverCross* crossp : m_coverCrosses) {
+                    crossp->unlinkFrBack();
+                    VL_DO_DANGLING(crossp->deleteTree(), crossp);
+                }
+                return;
+            }
 
             // Find the sample() method and constructor
             m_sampleFuncp = VN_CAST(m_memberMap.findMember(nodep, "sample"), Func);