machine_custom_init() runs once at boot, but its code and any tables it
uses were compiled into machine.c and so into the common bank. On the
SWTG018AS-V2.1.0 variant, whose init carries a 21-entry LED register
table, that overflows bank 0 by 0x66 bytes and main no longer links for
MACHINE_PCB_SWTG018AS_V2_1_0; any machine whose init grows can hit the
same wall. SDCC segment pragmas apply file-wide, so the hooks move to a
new machine_init.c compiled into BANK2, and the prototype becomes
banked. machine_check also compiles the new file per machine so the
hooks keep CI syntax coverage.
