Add "zstd" library during build to support "zstd" module in stdlib

[tianon]

See https://docs.python.org/3.14/library/compression.zstd.html (new in 3.14+)

• Fixes No module named '_zstd' #1085
• See also Test for zstd in Python 3.14+ official-images#20054

[tianon]

@yosifkit this part of the diff is because I went a little bit overboard to add support for "build-time-only" deps in non-slim Debian variants - I'm happy to refactor further if you'd like to avoid pushing these down into the list below 👍

[tianon]

(if we deal with docker-library/official-images#20054 first, we can close/reopen this and kick off a manual build on master to see it actually tested both directions)

[yosifkit]

You can see the expected, failing tests in https://github.com/docker-library/python/actions/runs/18357122459

Traceback (most recent call last):
  File "/tmp/test-dir/python-imports/./container.py", line 29, in <module>
    from compression import zstd
  File "/usr/local/lib/python3.14/compression/zstd/__init__.py", line 29, in <module>
    import _zstd
ModuleNotFoundError: No module named '_zstd'
	'python-imports' [4/8]...failed

[tianon]

Oh, apparently non-slim didn't technically need this:

#9 1.305 + apt-get install -y --no-install-recommends libzstd-dev
#9 1.312 Reading package lists...
#9 1.753 Building dependency tree...
#9 1.858 Reading state information...
#9 2.010 libzstd-dev is already the newest version (1.5.7+dfsg-1).
#9 2.010 libzstd-dev set to manually installed.
#9 2.010 0 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.

However, it's not explicitly part of buildpack-deps, only transitively, so I'm still convinced this is probably correct even though both blocks do ~nothing.