[iam-TJ]

"So you can’t transparently bridge Ethernet devices’ MAC addresses through a WiFi client interface. This is why we need hostapd."

I think that is incorrect. hostapd handles the authentication side of things, but 4addr tuples are controlled by 'struct wireless_dev.use_4addr', and can be set by 'ip link set type bridge_slave ... proxy_arp_wifi on', `iw dev ... 4addr on', and if using systemd-networkd, with slave interface's

  [Bridge]
  ProxyARPWiFi=yes

(and networkd doesn't need hostapd's bridge= option since networkd handles that aspect.)

Kernel then uses NL80211_IFTYPE_AP_VLAN and handles the proxy operation.

[0o_MrPatrick_o0]

It may be possible that this has changed. Last year I built a device and crashed into the bridging weirdness when I wanted to use upstream dhcp. There was/is some funkiness lurking with bridging wifi to Ethernet- in particular with broadcasts that traverse the bridge.