[Test] (Retrace): make a retrace under MOBILEGL_PIPE_VERIFY prove it armed, and give the lane a label

- A retrace that exports MOBILEGL_PIPE_VERIFY=1 at a library configured without
  -DMOBILEGL_PIPE_VERIFY=ON is a no-op: the frames still match their goldens and the
  case reports green having compared nothing. run_trace_case.cmake now demands the
  evidence whenever the variable is set to anything but 0/false - mobilegl.log must
  exist, must carry "MGPipe: verify armed", and must carry neither
  Fatal{PipeVerifyDiffer nor Fatal{UnmigratedPipeInput. With the variable unset the
  script is byte-for-byte the old one.
- The Fatal scan is not redundant with the replay's exit status:
  MOBILEGL_PIPE_VERIFY_FATAL=0 is the supported triage configuration, and there a
  divergence is logged and counted rather than aborted, so the run would finish 0
  with its own report sitting unread in the log.
- LABELS retrace on both registrations: the lane was selectable only by regex, so
  `ctest -L retrace --no-tests=error` - the spelling that reds a lane which
  registered nothing - could not be written at all.
- "verify": true on eight cases (the five 180s cases plus minecraft-1.21.4-in-world,
  minecraft-1.21.4-fabric-sodium-in-world and improved-transparency-minecraft-26.3),
  and --format github-verify-matrix over that subset: 16 entries, against the full
  lane's 77. The verify build compares at every verb boundary and again at every
  accessor read, which the design budgets at 5-10x, so the per-push job runs the
  subset and the full sweep is a phase-exit / workflow_dispatch run.
- The flag is validated in the manifest loader, not at the matrix, so a non-boolean
  or a verify case excluded from CI is a loud error in every consumer instead of a
  subset that is quietly one case short.

tools/trace_replay/run_trace_case.cmake

@@ -127,3 +127,60 @@ endif()
 if(NOT replay_result EQUAL 0)
     message(FATAL_ERROR "${TRACE_CASE_NAME} ${TRACE_BACKEND} trace replay failed with status ${replay_result}")
 endif()
+
+# --- MOBILEGL_PIPE_VERIFY: the third CI mode's own assertions (gates G3 and G8) --------------
+#
+# A retrace that exported MOBILEGL_PIPE_VERIFY=1 at a library which was never configured with
+# -DMOBILEGL_PIPE_VERIFY=ON is a no-op that looks exactly like a clean pass: the variable steers
+# nothing, the frames still match their goldens, and the case reports green having verified
+# nothing at all. The mode therefore has to prove it ran, and the only channel a `cmake -P` script
+# has for that is the library's own log.
+#
+# Three demands, all of them silent when MOBILEGL_PIPE_VERIFY is unset or "0", so an ordinary
+# retrace is untouched:
+#   * mobilegl.log exists - the replay wrote one, so the library was loaded and logging;
+#   * it carries "MGPipe: verify armed" - the comparator armed in THIS process;
+#   * it carries neither Fatal{PipeVerifyDiffer (a push/pull divergence, the thing the mode
+#     exists to find) nor Fatal{UnmigratedPipeInput (a backend read of a field the verb's fill
+#     table does not list - fixed by adding the row to MG_Pipe/FillPoints.def, never by marking
+#     the field sticky).
+# The Fatal check is not redundant with the replay's exit status: MOBILEGL_PIPE_VERIFY_FATAL=0 is
+# the supported triage configuration, and there the divergence is logged and counted rather than
+# aborted, so the run would otherwise finish 0 with its own report in the log.
+if(DEFINED ENV{MOBILEGL_PIPE_VERIFY} AND NOT "$ENV{MOBILEGL_PIPE_VERIFY}" STREQUAL "")
+    set(pipe_verify_case "${TRACE_CASE_NAME} ${TRACE_BACKEND}")
+    if("$ENV{MOBILEGL_PIPE_VERIFY}" STREQUAL "0" OR "$ENV{MOBILEGL_PIPE_VERIFY}" STREQUAL "false")
+        message(STATUS "MGPipe verify: MOBILEGL_PIPE_VERIFY=$ENV{MOBILEGL_PIPE_VERIFY}, no verify assertions for ${pipe_verify_case}")
+    elseif(NOT EXISTS "${mobilegl_log}")
+        message(FATAL_ERROR
+                "MOBILEGL_PIPE_VERIFY is set for ${pipe_verify_case} but the run wrote no ${mobilegl_log}, "
+                "so there is no evidence the comparator ever armed. A verify retrace with no library log "
+                "cannot be counted as a verify retrace.")
+    else()
+        file(READ "${mobilegl_log}" pipe_verify_log)
+        string(FIND "${pipe_verify_log}" "MGPipe: verify armed" pipe_verify_armed_at)
+        if(pipe_verify_armed_at EQUAL -1)
+            message(FATAL_ERROR
+                    "MOBILEGL_PIPE_VERIFY is set for ${pipe_verify_case} and the library never reported "
+                    "\"MGPipe: verify armed\". Either this libMobileGL.so was not built with "
+                    "-DMOBILEGL_PIPE_VERIFY=ON - in which case the whole verify retrace lane is comparing "
+                    "nothing - or the runtime knob never reached the process. Check that the VERIFY "
+                    "runtime artifact is the one unpacked at ${MOBILEGL_LIBRARY}.")
+        endif()
+        file(STRINGS "${mobilegl_log}" pipe_verify_fatals
+                REGEX "Fatal\\{(PipeVerifyDiffer|UnmigratedPipeInput)")
+        if(pipe_verify_fatals)
+            foreach(line IN LISTS pipe_verify_fatals)
+                message(STATUS "${line}")
+            endforeach()
+            list(LENGTH pipe_verify_fatals pipe_verify_fatal_count)
+            message(FATAL_ERROR
+                    "${pipe_verify_case}: ${pipe_verify_fatal_count} MGPipe Fatal(s) under "
+                    "MOBILEGL_PIPE_VERIFY. Fatal{PipeVerifyDiffer, \"<Field>@<Verb>\"} is a real push/pull "
+                    "divergence and is recorded, not silenced; Fatal{UnmigratedPipeInput, \"<Field>@<Verb>\"} "
+                    "is a missing row in MG_Pipe/FillPoints.def's class table - add it, regenerate, rerun "
+                    "(never mark the field sticky).")
+        endif()
+        message(STATUS "MGPipe verify: ${pipe_verify_case} armed, zero divergences, zero unmigrated reads")
+    endif()
+endif()