fix(q5_max): fix RAW_EPSIZE and via_command_kb hook conflict

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).

keyboards/keychron/common/keychron_raw_hid.h

@@ -63,3 +63,17 @@ enum {
     REPORT_RATE_GET,
     REPORT_RATE_SET,
 };
+
+/**
+ * Keyboard-level Raw HID extension hook.
+ *
+ * Called by kc_raw_hid_rx() for any command ID not handled by Keychron's own
+ * protocol (0xA0-0xAB range).  The default weak implementation returns false
+ * so that unrecognised commands fall through to VIA.
+ *
+ * Override this in your keymap to handle custom command IDs without
+ * conflicting with Keychron's via_command_kb() definition.
+ * Return true if the packet was fully handled (including calling
+ * raw_hid_send() if a reply is needed), false to let VIA process it.
+ */
+bool kc_raw_hid_rx_kb(uint8_t *data, uint8_t length);