Add txn-tester transaction/isolation bug-detection builders

[arcivanov]

What

Integrates txn-tester — a single image bundling three DBMS transaction /
isolation bug-detection tools, all built from source — into the non-latent
master:

Tool	Upstream
TROC	tcse-iscas/Troc
Fucci	Reverie4u/Fucci
APTrans	Paper-code-sigmod/APTrans

It is wired exactly like the WordPress eco-test (f_wordpress): the freshly
CI-built MariaDB docker-library image is run in a pod and the tools are pointed at
it over the pod-local loopback. The tools connect to MariaDB purely via
environment variables; this adds no MariaDB build of its own.

How

• scripts/docker-library-build.sh — retain a <image>-txntester tag on the
  amd64 image (next to the existing -wordpress tag) so the factory has the
  CI-built MariaDB to run against.
• master-nonlatent/master.cfg — getTxnTesterFactory() creates a pod, runs
  the CI-built MariaDB, then runs the three tools sequentially via podman and
  uploads their artifacts. Two builders:
  • light (amd64-rhelN-txntester, ~6–7 min) — fast per-build signal;
  • soak (amd64-rhelN-txntester-soak, ~30 min) — deep coverage, opt-in.
• f_dockerlibrary triggers the light builder on every saved-package build,
  and the soak builder instead on txn-prefixed branches (isTxnTesterBranch),
  mirroring the not/is-branch split used for Jepsen.
• constants.py / schedulers_definition.py — BUILDERS_TXN_TESTER[_SOAK]
  and s_txn_tester[_soak].

Bug signalling

The upstream tools never exit non-zero on a finding (TROC/Fucci loop until killed
by timeout and only log a BUG REPORT; APTrans only writes a check/ artifact
then exits 0). The image therefore maps a discovered inconsistency to the exotic
exit code 100, and the steps use decodeRC={0: SUCCESS, 100: FAILURE} so a
found transaction anomaly turns the build red and is distinguishable from an
infrastructural failure (image pull / classpath / connection), which keep their
own non-zero codes.

Image

Pulled from ghcr.io/arcivanov/txn-tester:0.0.2 (--pull=always), pinned for
reproducibility. It lives in its own repo for now; if MariaDB adopts it, only the
image reference changes (optionally adding an in-tree eco_build_images build).

Notes

• Validated with ./validate_master_cfg.sh -e DEV — all master configs check out.
• Heads-up: in local/CI testing, TROC and Fucci both report inconsistencies
  against stock MariaDB within a short run, so these builders will go red fairly
  often. That is the intended finding→red→triage behaviour; whether the reports
  are true anomalies or tool noise is worth review before treating red/green as a
  clean pass/fail signal.