EC618 base v5

EC618 base artifact set used by envelope producers to link compatible slots.

This is the universal base; the anchor record selects the console
UART. Slots link against base.elf; base.bin carries the stamped
base-id record the device checks before accepting a slot OTA.
The -api.json file records the reviewed peripheral export contract;
retained native APIs are not a claim of hardware qualification.
Distributed envelopes carry the complete matching AP image; a
flasher never downloads this release separately. To build an
envelope against this release: download the files into a directory
as base.elf/base.bin and run make ec618 EC618_BASE_DIR=<dir>.