qemu: json/stat/region channels, seeding, driver tests

- rootfs/sbin/init: the control bridge answers `@cat PATH` locally so the
  driver can read /tmp/warden-web-status.json out of the guest. That file
  has no trailing newline; the bridge adds one so the sentinel stays on
  its own line and the line-based reader never blocks.
- tests/qmp.py: wait_json/assert_json (dotted paths, eq/ne/contains/
  len_eq/len_ge/gt/lt), assert_stat off the FIFO's stats reply,
  capture_region, assert_region NAME [TOLERANCE] and assert_ocr. A
  tolerance other than the captured one, a reference box that does not
  fit the screendump, a missing reference or a missing tesseract is FATAL
  for that step and the run continues (flare-edge #147).
- tests/imgtools.py: P6 reader, crop, perceptual and structural hashes,
  compare, with a self-test.
- tests/test_qmp_drive.py: drive() with QMP and the control channel
  faked, pinning the per-step ok/fail/fatal contract.
- mkimage.sh SEED_DIR and ui-drive.sh --seed/--refs: settings fixtures
  staged into userdata before warden-ui starts, and a reference store
  handed to the driver.
- ci: the driver tests and the imgtools self-test run in qemu-tools.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013aHKWzT5EF86RFKRMtAv9n

.github/workflows/ci.yml

@@ -111,6 +111,14 @@ jobs:
           shellcheck -x qemu/*.sh qemu/tests/*.sh build/*.sh \
             qemu/rootfs/etc/warden-lib.sh qemu/rootfs/etc/rc \
             qemu/rootfs/sbin/init qemu/rootfs/init
+      - name: ui-drive driver and image tools (offline)
+        # qmp.py's drive() with QMP and the control channel faked, plus
+        # imgtools' self-test: the per-step ok/fail/fatal contract and the
+        # region reference math, no VM needed.
+        run: |
+          sudo apt-get install -y -qq python3-pil
+          python3 qemu/tests/imgtools.py selftest
+          python3 qemu/tests/test_qmp_drive.py
       - name: cache pinned busybox
         uses: actions/cache@v4
         with: