Artemis Milestone 1 complete: QMP socket added to all three QEMU

launch targets, three-arch verified

Makefile.starkernel: added -qmp unix:$QMP_SOCK,server=on,wait=off to
the amd64/aarch64/riscv64 qemu targets, matching the existing serial
chardev socket pattern exactly (same discoverability, same cleanup on
exit). Verified on all three architectures: QMP greeting arrives on
connect, qmp_capabilities handshake succeeds, device_add/device_del
round-trip correctly.

Real finding surfaced during device_add testing (recorded in
FABRIC-2.md's punch list for Milestone 2): the q35 machine's pcie.0
root bus doesn't support runtime PCI hotplug without a bridge --
Milestone 2's qemu-xhci USB controller needs to be present in the
static launch command, with USB devices hot-attached to its bus at
runtime, not the controller itself hot-added.

Also noted: g_doe_log_enabled's default-on per-tick heartbeat CSV
export was briefly mistaken for a hang during aarch64 verification --
it isn't one, just a large volume of routine diagnostic output before
reaching ok>. Not changing the source default; adopting HB-OFF
immediately after boot as the working pattern for the rest of this
punch list's dev work.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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