[docs] Config.transform docstring calls rsvd the only built-in, but nystrom is a built-in too

[luciferlive112116]

What

strategy/config.py's Config docstring (line 27) describes the transform knob as:

transform     : registry name ("rsvd", the only built-in) OR a Transform
                instance (the pluggable "core tech").

But rsvd is not the only built-in — nystrom is registered as a built-in too.

Evidence (code contradicts the docstring)

• strategy/transforms.py:227-228 registers two built-ins at import time:

  for _cls in (RandomizedSVDTransform, NystromTransform):
      register_transform(_cls.name, _cls)

  so available() returns ['nystrom', 'rsvd'].
• Every other contributor-facing surface already says both: transforms.py's module docstring ("Built-in transforms: rsvd … and nystrom") and strategy/README.md:44 ("rsvd and nystrom are the built-in transforms").

config.py is the lone straggler — nystrom was added as a built-in in PR #194 and this docstring was never updated.

Fix

Correct the parenthetical to name both built-ins, and add a CPU-only regression test pinning the docstring to the real built-in set so a future built-in can't silently leave it stale.