engine: add the export family to the C ABI facade; mp4 transcode

- oakengine_export_render drives ExportTask synchronously (offline
  render + encode) with a progress callback, codec probing, and a
  thread-local error channel; exporter.h keeps clear of the visibility
  macro header
- oak-cli transcode now defaults to mp4 (H.264/AAC) with --format ppm
  keeping the raw output path
- two real concurrency bugs found by the facade's own test: ExportTask
  deadlocks when start()ed synchronously (queued conform handshake
  needs an event loop), and the progress callback must be captured by
  value because it fires on the task thread

cli/verify_transcode_mp4.cmake

@@ -0,0 +1,44 @@
+# Oak - Non-Linear Video Editor
+# Copyright (C) 2026 Oak Team
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Post-check for the oak_cli_transcode test: the MP4 exists and ffprobe
+# reports an h264 video stream (invoked with -DOUT=<file>).
+
+if (NOT EXISTS "${OUT}")
+    message(FATAL_ERROR "transcode output missing: ${OUT}")
+endif ()
+
+execute_process(
+        COMMAND ffprobe -v error -select_streams v:0 -show_entries stream=codec_name,width,height -of csv=p=0 "${OUT}"
+        OUTPUT_VARIABLE probe_out
+        RESULT_VARIABLE probe_rc
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+if (NOT probe_rc EQUAL 0)
+    message(FATAL_ERROR "ffprobe failed on ${OUT}")
+endif ()
+
+string(FIND "${probe_out}" "h264" h264_pos)
+if (h264_pos EQUAL -1)
+    message(FATAL_ERROR "no h264 stream in ${OUT} (ffprobe: ${probe_out})")
+endif ()
+
+string(FIND "${probe_out}" "960,540" size_pos)
+if (size_pos EQUAL -1)
+    message(FATAL_ERROR "unexpected dimensions in ${OUT} (ffprobe: ${probe_out})")
+endif ()
+
+message(STATUS "transcode mp4 verified: ${OUT} (${probe_out})")