Implement SCSI WRITE(10), closing the graph's highest-leverage blocker

Direct mirror of the existing READ(10) implementation (FABRIC-3.md §F.1),
data direction flipped: new XHCI_XFER_BOT_DATA_OUT/XHCI_NEXT_ACTION_BOT_DATA_OUT
states, xhci_bot_send_write10()/xhci_bot_write_block()/xhci_bot_write_data_out()
in xhci.c, new SCSI_CMD_WRITE10 opcode and BOT_CMD_WRITE10/BOT_TUR_CHAIN_WRITE10
enum values. usb_blk_write() in blkio_usb.c is real now, no longer the
BLKIO_ENOSUP stub. read_only flips to 0 in blkio_info() now that it's proven.

Verified live end-to-end on all three architectures with a genuine cold-reboot
round-trip (not just a same-session read): BLK-CONFIRM-FORMAT's BAM/reloc
writes and an explicit block content write both completed via clean WRITE10
cycles (CSW PASS), and the written byte read back correctly after a full
kernel rebuild + fresh boot -- amd64=65, aarch64=170, riscv64=201, each at
LBN 32734 on a disposable usbwrite-test.img attached via QEMU usb-storage.

Added Makefile.starkernel's QEMU_EXTRA (empty by default, no behavior change)
to attach the disposable test image for this validation; the drive must be
hotplugged via QMP after boot reaches ok>, not attached at QEMU launch --
attaching before xhci_bringup()'s controller reset means no fresh Port
Status Change event fires (see project_xhci_milestone_2d_polling memory).

Found and reported, not fixed, during testing: EMPTY-BUFFERS
(empty_all_buffers(), block_words.c) does not implement standard Forth-79
semantics -- it force-writes zero to every block on every attached device
instead of discarding cache assignments. This corrupted disk/artemis.img
during an earlier test run; restored from git, confirmed byte-identical.
Avoided in the final validation runs (detach/reattach used instead to force
a fresh read).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019ZGkimpfyh63EZyRkNbkPD

Makefile.starkernel

@@ -731,6 +731,14 @@ OVMF_VARS_RO := /usr/share/OVMF/OVMF_VARS_4M.fd
 # Usage: make -f Makefile.starkernel qemu QEMU_DISPLAY=none   # headless/CI
 QEMU_DISPLAY ?= gtk
 
+# QEMU_EXTRA — extra raw qemu-system-* args appended verbatim to every arch's
+# qemu invocation below. Empty by default (no behavior change for a normal
+# boot). Existing use: attaching a disposable USB test image for WRITE(10)
+# validation, e.g.
+#   QEMU_EXTRA="-drive if=none,id=usbtest,file=disk/usbwrite-test.img,format=raw \
+#               -device usb-storage,bus=xhci0.0,drive=usbtest"
+QEMU_EXTRA ?=
+
 qemu: all
 ifeq ($(ARCH),amd64)
 	@echo "=== Building ISO (amd64) ==="
@@ -782,6 +790,7 @@ ifeq ($(ARCH),amd64)
 		-chardev socket,id=cserial,path=$$SERIAL_SOCK,server=on,wait=off,logfile=$$LOG \
 		-serial chardev:cserial \
 		-qmp unix:$$QMP_SOCK,server=on,wait=off \
+		$(QEMU_EXTRA) \
 		-display $(QEMU_DISPLAY) \
 		-no-reboot; \
 	kill $$TAILPID 2>/dev/null; wait $$TAILPID 2>/dev/null || true; \
@@ -865,6 +874,7 @@ else ifeq ($(ARCH),aarch64)
 		-chardev socket,id=cserial,path=$$SERIAL_SOCK,server=on,wait=off,logfile=$$LOG \
 		-serial chardev:cserial \
 		-qmp unix:$$QMP_SOCK,server=on,wait=off \
+		$(QEMU_EXTRA) \
 		-display $(QEMU_DISPLAY) \
 		-m 4096 \
 		-no-reboot \
@@ -953,6 +963,7 @@ else ifeq ($(ARCH),riscv64)
 		-chardev socket,id=cserial,path=$$SERIAL_SOCK,server=on,wait=off,logfile=$$LOG \
 		-serial chardev:cserial \
 		-qmp unix:$$QMP_SOCK,server=on,wait=off \
+		$(QEMU_EXTRA) \
 		-display $(QEMU_DISPLAY) \
 		-no-reboot \
 		-d guest_errors; \