feat(render): Vulkan campaign V11 step 4 — retire GL from CI and gate scripts

Closes out the GL deletion by fixing the CI workflow and developer gate
scripts that still assumed a GL arm existed to compare against, build,
or select via ACDREAM_RENDER_BACKEND.

.github/workflows/headless-portability.yml: linux-graphical's "Verify
actionable unsupported-driver gate" step is deleted outright — it ran
the deleted `ui-studio` CLI verb (Studio was removed at Commit 1) to
prove the GL capability gate rejects Mesa's llvmpipe driver, and there
is no more GL capability gate for any driver to pass or fail. Its test
filter dropped two dead entries (GraphicalCapabilityRequirementsTests,
deleted at Commit 2; StudioWindowTests, already gone). Its package
contract check dropped the libcimgui.so assertion (ImGui's native
bridge, deleted at Commit 1). linux-vulkan's explanatory comment, which
described GL's rejection as the reason no cross-backend pixel diff runs
in CI, is rewritten to explain there is no GL arm left at all. Two dead
src/AcDream.UI.ImGui/** path triggers (that project no longer exists)
are removed from both the pull_request and push filters.

tools/run-backend-differential-gate.ps1 and its dedicated route file
tools/connected-backend-differential.route.txt are deleted: the whole
script's purpose was comparing a GL launch against a Vulkan launch of
the same route, and there is no second arm left to compare. Single-arm
regression checking already exists via run-offline-pixel-gate.ps1's
-Baseline mechanism.

tools/run-portal-churn-soak.ps1 is simplified rather than deleted: its
repeated-portal-churn methodology (within-arm capture comparison,
memory/entity/GPU trend analysis) has value independent of the
GL-versus-Vulkan question it was built to answer for issues #256/#257
before V11. -Backends now defaults to @('vulkan') alone; the doc
comments are rewritten from "step 0 discriminator, run before V11" to
an ongoing single-arm regression soak.

tools/run-offline-pixel-gate.ps1 drops its now-nonfunctional -Backend
parameter (ACDREAM_RENDER_BACKEND has read zero call sites since
RuntimeOptions.RenderBackend was removed at Commit 2 — passing -Backend
gl silently launched Vulkan anyway) along with its GL-escape-hatch
example and every comment that referenced the now-deleted differential
gate. tools/run-connected-world-lifecycle-gate.ps1,
tools/run-offline-vulkan-capture.ps1, and
tools/run-repeat-connected-gate.ps1 keep their (harmless, already
no-op) ACDREAM_RENDER_BACKEND set/clear lines but have their
now-inaccurate "escape hatch" / "GL run" comments corrected to state
plainly that the variable is unread and the line is kept only for the
historical record.

No .cs files touched; `dotnet build AcDream.slnx -c Release` unaffected
(0 warnings, 0 errors, matching the prior commit's build).

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

tools/run-repeat-connected-gate.ps1

@@ -112,11 +112,12 @@ sleep 20000
     $env:ACDREAM_RETAIL_UI = '1'
     $env:ACDREAM_UI_PROBE_SCRIPT = Join-Path $dir 'probe.txt'
     $env:ACDREAM_AUTOMATION_ARTIFACT_DIR = $dir
-    # Campaign V slice V10: deliberately cleared rather than set. The blank-world
-    # defect this gate exists for is a property of whichever backend the client
-    # actually starts, so the gate must launch the process default -- Vulkan
-    # after the cutover -- and an ambient override from the caller's shell would
-    # silently make it a GL run wearing the default's report.
+    # Campaign V slice V10 deliberately cleared this rather than setting it,
+    # because the blank-world defect this gate exists for is a property of
+    # whichever backend the client actually starts, and the gate had to launch
+    # the process default -- Vulkan after the cutover, and the ONLY backend
+    # since Campaign V slice V11 deleted the GL arm this variable used to
+    # select. The clear is now a no-op kept for the record.
     Remove-Item Env:\ACDREAM_RENDER_BACKEND -ErrorAction SilentlyContinue
 
     $log = Join-Path $dir 'client.log'