[api] Second cleanup pass over the decode generator

Repeated fields encode their elements through one encode_element() hook instead of two
isinstance ladders, the fixed32 precomputed tag path owns its own guard, the generated
switches drop the dead default case, StringRef takes the byte pointer directly, the
three hand written tag expressions in proto.h go through proto_tag(), and stale comments
about the previous decode design go. The compiled functions are unchanged.

tests/benchmarks/components/api/bench_proto_proxy.cpp

@@ -249,7 +249,7 @@ static APIBuffer build_infrared_rf_transmit_wire() {
   std::memcpy(bytes + len, packed, packed_len);
   len += packed_len;
   // field 6: modulation = 1 (non-zero so it's actually emitted and exercises
-  // decode_varint for this field, matching the documented layout above).
+  // decode_field for this field, matching the documented layout above).
   put_byte(0x30);
   put_varint(1);