Proposed external project scaffold

.github/scripts/expected-versions.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# CI-only: print the version strings the build is expected to embed, as `key=value` lines suitable for
+# appending to $GITHUB_OUTPUT. Used to assert requirement 4 (dual-version reporting).
+#
+#   mrtrix=<...>   replicates MRtrix3's cmake/FindVersion.cmake exactly: `git describe --abbrev=8
+#                  --dirty --always` when a tag is reachable, else the MRtrix3 base version.
+#   project=<...>  the project version recorded by splice-project.sh (git describe of the project src);
+#                  this is exactly what the build embeds via cmake/project_version.cpp.in.
+#
+# Usage: expected-versions.sh <mrtrix3_source_dir>
+set -euo pipefail
+
+MR_SRC="$1"
+
+tag="$(git -C "$MR_SRC" describe --abbrev=0 2>/dev/null || true)"
+commit="$(git -C "$MR_SRC" describe --abbrev=8 --dirty --always 2>/dev/null || true)"
+if [ -n "$tag" ] && [ -n "$commit" ]; then
+  mrtrix="$commit"
+else
+  # Fall back to the MRtrix3 base version declared in its top-level project().
+  mrtrix="$(grep -m1 -oE 'VERSION [0-9]+\.[0-9]+(\.[0-9]+)?' "$MR_SRC/CMakeLists.txt" | awk '{print $2}')"
+fi
+
+project=""
+if [ -f cmake/spliced_project_version.txt ]; then
+  project="$(head -n1 cmake/spliced_project_version.txt)"
+fi
+
+echo "mrtrix=$mrtrix"
+echo "project=$project"

.github/scripts/shape-scenario.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# CI-only: shape the spliced demo tree into one of the test scenarios. All demo-specific knowledge
+# (command names, the demo's MRtrix3 dependencies) lives here, inside .github/, so the template proper
+# stays project-agnostic.
+#
+# Environment:
+#   CMDFILE      none | subset | all | pydep -> what to write into cmake/mrtrix3_commands.txt
+#                  (pydep lists a single MRtrix3 *Python* command, named by PYDEP_CMD; this exercises
+#                   the "Python dependency -> build the full C++ set" behaviour)
+#   PYDEP_CMD    name of the MRtrix3 Python command to request when CMDFILE=pydep (default dwishellmath)
+#   DROP_PYTHON  "true" to remove the demo's Python command (yields a no-Python, single-binary project)
+#   MULTI        "true" to add a second C++ command. The demo's own core is split across a header and a
+#                COMPILED source (cpp/core/algo/geometric_mean.cpp), shared by both C++ commands; with
+#                more than one command sharing it the build collects that core into a branded
+#                lib<project>.so (over and above the MRtrix3 core's libmrtrix-core.so). A single command
+#                instead compiles the same core straight into the binary (no project library).
+set -euo pipefail
+
+: "${CMDFILE:=none}"
+
+: "${PYDEP_CMD:=dwishellmath}"
+
+case "$CMDFILE" in
+  none)   : > cmake/mrtrix3_commands.txt ;;
+  subset) printf 'mrconvert\nmrcalc\nmrmath\n' > cmake/mrtrix3_commands.txt ;;
+  all)    printf '*\n' > cmake/mrtrix3_commands.txt ;;
+  pydep)  printf '%s\n' "$PYDEP_CMD" > cmake/mrtrix3_commands.txt ;;
+  *) echo "shape-scenario: unknown CMDFILE='$CMDFILE'" >&2; exit 2 ;;
+esac
+echo "shape-scenario: MRtrix3 command file ($CMDFILE):"; sed 's/^/    /' cmake/mrtrix3_commands.txt
+
+if [ "${DROP_PYTHON:-}" = "true" ]; then
+  rm -rf python/demo_code/commands/demo_python python/demo_code/commands/demo_python.py
+  echo "shape-scenario: removed the demo Python command (no-Python project)"
+fi
+
+if [ "${MULTI:-}" = "true" ]; then
+  # A second C++ command that shares the demo's compiled core (geometric_mean.cpp, spliced from the
+  # demo source). Two commands sharing that real core source is what makes the build collect it into a
+  # branded lib<project>.so - no CI-synthesised core source is needed any more.
+  cp cpp/cmd/demo_cpp.cpp cpp/cmd/demo_cpp2.cpp
+  echo "shape-scenario: added demo_cpp2 (second C++ command sharing the demo's compiled core)"
+  if [ -f cpp/core/algo/geometric_mean.cpp ]; then
+    echo "shape-scenario: project compiled core present: cpp/core/algo/geometric_mean.cpp"
+  else
+    echo "shape-scenario: ERROR: expected demo compiled core cpp/core/algo/geometric_mean.cpp not found" >&2
+    echo "shape-scenario:   (is DEMO_CODE_REF pinned to a revision that splits geometric_mean.h/.cpp?)" >&2
+    exit 1
+  fi
+fi