[api] Hoist pos_ to local in encode_varint_raw_64 to avoid reload per byte

Use a __restrict__ local pointer for the varint write loop so the
compiler can keep it in a register instead of reloading pos_ from
memory on each iteration. Eliminates the store→load dependency
chain that was causing 7 load/store pairs for a typical 48-bit
BLE address varint.

esphome/components/api/proto.h

@@ -224,8 +224,6 @@ class ProtoWriteBuffer {
     this->encode_varint_raw_slow_(value);
   }
   void encode_varint_raw_64(uint64_t value) {
-    // Use __restrict__ so the compiler knows pos doesn't alias this->
-    // and can keep it in a register across the loop.
     uint8_t *__restrict__ pos = this->pos_;
     while (value > 0x7F) {
       *pos++ = static_cast<uint8_t>(value | 0x80);