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

.gitea/workflows/unit-tests.yml

@@ -40,7 +40,7 @@ jobs:
     # bumping the tag in scripts/ci/build_builder_image.sh AND here, in one
     # commit -- see that script's header.
     container:
-      image: gitea.tourolle.paris/dtourolle/sae-builder-cpu:v1
+      image: gitea.tourolle.paris/dtourolle/sae-builder-cpu:v2
 
     steps:
       - name: Checkout repository
@@ -73,8 +73,8 @@ jobs:
           # line above and the image that actually landed disagree, which is
           # exactly the drift the pinning exists to prevent -- so it fails the
           # job rather than building against an unknown toolchain.
-          [ "$SAE_BUILDER_VERSION" = "v1" ] || {
-            echo "image reports version '$SAE_BUILDER_VERSION', workflow pins v1" >&2
+          [ "$SAE_BUILDER_VERSION" = "v2" ] || {
+            echo "image reports version '$SAE_BUILDER_VERSION', workflow pins v2" >&2
             exit 1
           }