feat(frank-tts-engine): synthesize on the gfx1151 iGPU via Vulkan

The NPU path is a dead end for this payload (verdict:
report/tts-npu-spike-verdict-2026-10-19.md); the iGPU is the accelerator that
exists on this box today, and it is already shared with frank-dragoon.

- Containerfile.engine: -DGGML_VULKAN=ON, plus the three packages the door
  needs to configure — libvulkan-dev, spirv-headers, and glslc, which Ubuntu's
  glslang-tools does NOT ship (find_package(Vulkan COMPONENTS glslc REQUIRED)
  fails without it) — and libvulkan1 + mesa-vulkan-drivers (the RADV ICD) at
  runtime. No NVIDIA ICD on purpose: one Vulkan device in the enumeration is
  what makes the NAME Vulkan0 safe to write down.
- tts-engine-start.sh: QWENTTS_DEVICE -> --device, default Vulkan0. A value
  that is set but EMPTY passes no flag at all, which is the CPU rollback; that
  is why the default is written as an -v test and not with :=, because := also
  fires on an empty value and would silently undo the rollback.
- quadlet: AddDevice=/dev/dri + GroupAdd=keep-groups (a rootless container that
  drops its supplementary groups enumerates no render device). The node is not
  named: this host is hybrid, renderD128 is the NVIDIA card and renderD129 the
  amdgpu iGPU.
- env: QWENTTS_DEVICE=Vulkan0 plus the device law in the header, including the
  rollback line.
- tests: the two CPU-only assertions now assert the Vulkan shape, and a new one
  pins the rollback (empty device => no --device in argv). 451/451 green.

Measured before writing any of it: the configure in the digest-pinned base
answers "Found Vulkan: ... found components: glslc" and "Including Vulkan
backend"; gate 1 (RADV GFX1151 visible to a rootless container off this base)
is recorded in the verdict report.

scripts/frank-tts-engine.env

@@ -27,6 +27,16 @@
 # below stop describing the process you are running. Nothing in this fleet
 # pins this container; that is a decision, not an oversight.
 #
+# THE DEVICE LAW (since the 2026-10-19 Vulkan cutover). Synthesis computes on
+# the gfx1151 iGPU — the SAME card frank-dragoon serves. It is a shared device,
+# never a dedicated one, so two numbers matter and neither is what `rocm-smi`
+# prints first: the memory the container may use is the RADV heap inside the
+# GTT-backed aperture (the ~2 GiB DRM carve-out is not the working set), and the
+# timing that matters is dragoon's, measured WITH a synthesis in flight. The
+# rollback is one line: set QWENTTS_DEVICE empty, and the entrypoint passes no
+# --device at all — the pre-iGPU CPU shape, no rebuild, no restart of anything
+# else on the box.
+#
 # THE REGISTRY LAW (why the unit has an ExecStartPost at all). Registrations live
 # in memory only, so a restart loses them, and re-registration alone does NOT warm
 # a voice — the warm pass has to synthesize something. Without it the façade
@@ -103,6 +113,17 @@ QWENTTS_LANG=auto
 # round-trip outside it) is what absorbs the queue.
 QWENTTS_MAX_BATCH=1
 
+# The compute device, by ggml device NAME. `Vulkan0` is the gfx1151 iGPU: the
+# image carries the Mesa (RADV) ICD and no NVIDIA ICD, so this container's
+# Vulkan enumeration has exactly one entry and it is the iGPU. The unit hands
+# the container /dev/dri and keeps the render group; on this hybrid host the
+# iGPU's render node is renderD129 and renderD128 is the NVIDIA card, which is
+# why the unit passes the directory and lets the driver resolve the node.
+#
+# ROLLBACK: an empty value means no --device flag at all — the CPU shape this
+# endpoint shipped with until 2026-10-19. One line, no rebuild.
+QWENTTS_DEVICE=Vulkan0
+
 # --- the boot warmer (dankclaw/voiceservice/tts-engine-warm.sh) --------------
 # These three are read by the warm hook, not by tts-server. They live here
 # because the hook is exec-ed into this container and therefore inherits this