Artemis Milestone 2e: virtual USB thumb-drive hotplug test + block-layout decisions

Verified live on amd64: booted with the xHCI controller present but no USB
device attached (no Port Status Change at ok>), then hotplug-attached a
virtual USB thumb drive via QMP (usb-storage on xhci0.0, backed by
disk/usb-thumbdrive-test.img) and got an immediate port status change
event -- the real connect trigger Milestone 2e's PORTSC handling will
consume next.

Confirmed blk_subsys_attach_device() (src/block_subsystem.c) is already
the correct integration point for USB -- it already appends a new device
to the end of the existing LBN chain, matching the intended design.
Documented the remaining gaps: no blkio_usb.c backend yet, no hot-detach
path in the device chain yet.

Decided the on-drive layout for USB thumb drives: GPT-partitioned (unlike
artemis.img's whole-device StarForth header), ~1GB metadata partition +
remainder for blocks, 16GB reference drive size, sizing tentative. No GPT
parser exists in kernel code yet -- new prerequisite work for Milestone
2h/3, not blocking current 2e work.

disk/usb-thumbdrive-test.img added as a tracked test fixture, per this
repo's standing convention that virtual disk/thumb-drive images used for
testing are committed, not left in scratchpad.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HZ8kNoTuP63pbQtro4qvrm

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