ci: build flex-arm-wheels artifact (draft, do not merge)

[markzporter]

Draft — opened to trigger CI, not for merge.

build-flex-arm-wheels.yml only runs on workflow_dispatch, push to main/tags, or
pull_request to main. Every flex-arm-wheels artifact in the repo (16 of them, across
all branches) is now expired=true under retention-days: 14 — the newest was
2026-07-29, which lapsed 08-12. A fresh build is the only way to get dist_arm/ for
./deploy.sh, and this PR is the trigger that doesn't require admin rights.

This branch touches pyproject.toml, src/**, uv.lock, Dockerfile.build,
scripts/artifact_manifest.py, and packages/flex-acceptance-contract/**, all of which
are in the workflow's path filter.

Once the run is green the artifact gets pulled into dist_arm/ to deploy the connector to
Flex AcOpenTronsFlex01 (FLXA1020240103001). Close this without merging afterwards
unless you actually intend to land the branch.

🤖 Generated with Claude Code

[markzporter]

Artifact built and consumed: run 32172642429 produced flex-arm-wheels, verified locally as flex-0.9.1-ot9.0.0-py3.12-aarch64-b8db706b60ea and deployed to AcOpenTronsFlex01. Closing without merging, as intended.