Two build errors:
1. RAW_EPSIZE undeclared — usb_descriptor.h is not in scope when
keymap.c is compiled through Keychron's build path. Replace all
uses with HID_PACKET_SIZE (= 32), now defined in hid_protocol.h.
2. via_command_kb duplicate symbol — keychron_raw_hid.c already defines
via_command_kb (non-weak) so we cannot redefine it in keymap.c.
Fix by adding a weak kc_raw_hid_rx_kb() extension hook to
keychron_raw_hid.c (following the same pattern as kc_rgb_matrix_rx).
kc_raw_hid_rx() now calls this hook from its default case instead of
returning false directly. The keymap overrides kc_raw_hid_rx_kb() to
handle our custom HID command range (0x40-0x7E).