rsz: use cell id() tie-break for stable buffer-size sort

Per review, tie-break on LibertyCell::id() rather than the cell name: an
integer compare instead of constructing a string per comparison. This matches
the existing buffer comparator in Resizer.cc (findFastBuffers), which already
tie-breaks equal-key cells on id(). String compares are the most expensive
tie-break.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

src/rsz/src/Rebuffer.cc

@@ -1500,15 +1500,10 @@ void Rebuffer::init()
   }
 
   std::ranges::sort(buffer_sizes_, [=](BufferSize a, BufferSize b) {
-    const float cin_a = bufferCin(a.cell), cin_b = bufferCin(b.cell);
-    if (cin_a < cin_b) {
-      return true;
-    }
-    if (cin_b < cin_a) {
-      return false;
+    if (bufferCin(a.cell) != bufferCin(b.cell)) {
+      return bufferCin(a.cell) < bufferCin(b.cell);
     }
-    // buffer_fast_sizes_ is unordered; tie-break for stable output.
-    return std::string_view(a.cell->name()) < std::string_view(b.cell->name());
+    return a.cell->id() < b.cell->id();
   });
 
   buffer_sizes_index_.clear();

src/rsz/src/RepairDesign.cc

@@ -647,16 +647,10 @@ void RepairDesign::findBufferSizes()
                    resizer_->buffer_fast_sizes_.end()};
   std::ranges::sort(buffer_sizes_,
                     [=](sta::LibertyCell* a, sta::LibertyCell* b) {
-                      const float cin_a = bufferCin(a), cin_b = bufferCin(b);
-                      if (cin_a < cin_b) {
-                        return true;
+                      if (bufferCin(a) != bufferCin(b)) {
+                        return bufferCin(a) < bufferCin(b);
                       }
-                      if (cin_b < cin_a) {
-                        return false;
-                      }
-                      // buffer_fast_sizes_ is unordered; tie-break for stable
-                      // output.
-                      return std::string_view(a->name()) < std::string_view(b->name());
+                      return a->id() < b->id();
                     });
 }