fix(ci): sae-builder-cpu could not configure this repository at all

v1 has no libfftw3. SAE_SCENE_XGB is on by default and pulls the learned
scene-boundary detector into result_sink, whose find_library(FFTW3_LIB
fftw3 REQUIRED) at CMakeLists.txt:377 runs at CONFIGURE time -- so cmake
fails before any target is chosen, and building only `sae_tests` does not
get you past it. The image predates the detector; nothing rebuilt it when
the detector landed, which is precisely the gap DP-007's own register row
records.

-DSAE_SCENE_XGB=OFF would also have made the configure pass. It is the
wrong fix and worth saying why: CI is the only place these tests run, so
a flag that silences a subsystem there silences it everywhere, and the
job would have gone green over a detector nobody was compiling.

The image's self-check gains a line, because an image that ships a
library CMake cannot find the way CMake looks for it is the failure this
block exists to catch. FFTW is checked as a bare .so rather than through
pkg-config, since find_library is what CMakeLists.txt actually calls --
checking fftw3.pc would pass on an image whose libfftw3.so was absent.

Tag bumped v1 -> v2 here, in build_builder_image.sh and in the workflow's
run-time assert, in one commit, as this image's contract requires. v2 is
built and published. Rehearsed in the image before pinning it: configure,
build and 151 tests now run.

TRACES: DP-007 | PR-004

scripts/ci/build_builder_image.sh

@@ -5,9 +5,9 @@
 # TRACES: DP-007 | PR-004
 #
 # Usage:
-#   scripts/ci/build_builder_image.sh                  # build only, tag v1
+#   scripts/ci/build_builder_image.sh                  # build only, tag v2
 #   scripts/ci/build_builder_image.sh --push           # build and push
-#   scripts/ci/build_builder_image.sh --tag v2 --push  # bump the pinned tag
+#   scripts/ci/build_builder_image.sh --tag v3 --push  # bump the pinned tag
 #   scripts/ci/build_builder_image.sh --no-cache       # force a clean rebuild
 #
 # The tag is the contract with CI. .gitea/workflows/unit-tests.yml names an
@@ -34,7 +34,10 @@ DOCKERFILE="Dockerfile.builder-cpu"
 # .gitea/workflows/unit-tests.yml; the workflow asserts at run time that the
 # image it landed in reports this same version, so a drift shows up as a failed
 # job rather than as a build against the wrong toolchain.
-TAG="v1"
+#
+# v2 adds libfftw3-dev: the learned scene-boundary detector's find_library is
+# REQUIRED at configure time, so v1 cannot configure this repository at all.
+TAG="v2"
 
 PUSH=0
 EXTRA_ARGS=()