CI: add a 32-bit test job (armv7, emulated)

[ThomasWaldmann]

We have big-endian coverage (s390x) now, but no 32-bit platform: all our other CI
machines are 64-bit, so nothing exercises borg where a pointer, size_t,
Py_ssize_t and off_t are 32 bits wide.

Why armv7: 32-bit x86 desktops are basically gone, but 32-bit ARM is not — it
is what borg users on Raspberry Pi OS 32-bit and on quite some NAS / router /
embedded boxes run. It also has two properties x86 does not have: it does not
tolerate unaligned memory accesses everywhere, and plain char is unsigned on
ARM (signed on x86) — a classic C portability trap. GitHub's arm64 runners cannot
execute 32-bit ARM code natively, so this is emulated (qemu user-mode) regardless.

Why trixie: borg needs OpenSSL >= 3.2, so debian bookworm and its OpenSSL 3.0
are out. trixie has OpenSSL 3.5.6 and python 3.13 for armhf.

The job is modelled on bigendian.yml and has the same trigger policy: native /
format relevant paths on PRs, weekly (Wednesdays, one hour after the s390x job so
the two emulated jobs do not start at the same time), plus workflow_dispatch.

It runs the word size sensitive tests (chunkers, crypto, compress, hashindex,
item, repoobj, repository, archive, cache, digests, msgpack, time) and then the
cross-architecture interoperability test: a repository written on the emulated
32-bit machine is checked, extracted and written to by the 64-bit runner and the
other way round, and both sides must produce identical chunk IDs.

scripts/endian_interop_test.py is reused unchanged (its labels are just names
for the two sides); only its docstring mentions the second use now.

Note on runtime: armv7l has fewer wheels on PyPI than s390x — only blake3 ships
one, so PyYAML, msgpack and backports-zstd (which vendors the whole zstd library)
are compiled under emulation. The .pip-cache-armv7 cache keeps that to the
first run and to dependency bumps.

Not tested beyond YAML validity and the pytest target list — the first run on
GitHub will show the real emulated runtime.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.25%. Comparing base (2491d56) to head (d69838c).
⚠️ Report is 30 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10211      +/-   ##
==========================================
+ Coverage   87.20%   87.25%   +0.05%     
==========================================
  Files         102      102              
  Lines       18394    18430      +36     
  Branches     2829     2832       +3     
==========================================
+ Hits        16040    16082      +42     
+ Misses       1644     1642       -2     
+ Partials      710      706       -4     

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