Add the parallel set of CI jobs that exercise the cmakev2 path end-to-end
via the IDF_BUILD_V2 shim. All jobs are gated on the `buildv2` MR label
with `allow_failure: true` so unrelated MRs do not pay for the extra
build matrix.
- `.gitlab/ci/build.yml`: add the buildv2 child build pipeline that
re-runs the full app build matrix with IDF_BUILD_V2=y, plus the
Windows buildv2 build job. The generator script injects IDF_BUILD_V2
and a PIPELINE_COMMIT_SHA suffix into each child job and broadens
target_test job filters so v1 and buildv2 artifacts upload to distinct
s3 cache paths and each child pipeline tests its own binary.
- `.gitlab/ci/host-test.yml`: add the QEMU (esp32, esp32c3) and Linux
pytest buildv2 jobs.
- `.idf_build_apps.toml`: parameterize build_dir with BUILDV2_DIR_SUFFIX
(set to _v2 only on the buildv2 pipeline).
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