Merge origin/inline-varint-fast-path into integration branch

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

esphome/components/api/proto.cpp

@@ -16,6 +16,19 @@ void ProtoByteBuffer::grow_(size_t n) {
   this->capacity_ = n;
 }
 
+uint32_t ProtoSize::varint_slow_(uint32_t value) {
+  // value is guaranteed >= 128 here (fast path handled inline)
+  if (value < 16384) {
+    return 2;  // 14 bits
+  } else if (value < 2097152) {
+    return 3;  // 21 bits
+  } else if (value < 268435456) {
+    return 4;  // 28 bits
+  } else {
+    return 5;  // 32 bits (maximum for uint32_t)
+  }
+}
+
 #ifdef USE_API_VARINT64
 optional<ProtoVarInt> ProtoVarInt::parse_wide(const uint8_t *buffer, uint32_t len, uint32_t *consumed,
                                               uint32_t result32) {