Merge LogCpTransformer into LogTransformer with C parameter

[adityaanikam]

Closes #957

Summary

Merges LogCpTransformer into LogTransformer as approved in the issue
discussion. LogTransformer gains a C parameter ("auto" / int / float /
dict) mirroring LogCpTransformer's exact semantics, defaulting to C=0 so
existing LogTransformer() usage is unchanged. LogCpTransformer becomes a
thin subclass defaulting to C="auto", noted as being consolidated into
LogTransformer in favor of LogTransformer(C=...).

Backward compatibility

• LogTransformer()'s behavior and error message/timing are unchanged:
  fit-time validation still fires exactly when C resolves to 0 (the new
  default), with the original message.
• LogCpTransformer's numerical behavior and C_ computation are unchanged.

Things worth flagging

1. LogCpTransformer's base= error message lost its dynamic
   Got {base} instead. suffix — it now uses LogTransformer's original
   plain message, since that's the class whose wording was promised
   unchanged. Updated the corresponding test to match.
2. LogCpTransformer gets its own _more_tags()/__sklearn_tags__()
   overrides resetting to the un-xfailed tags, so it doesn't silently
   inherit LogTransformer's zero-value xfail exemptions that it never
   needed (its "auto" default always finds a valid shift).
3. There's no existing convention in this codebase for deprecating a class
   (searched — no precedent), so I kept the notice to a plain docstring
   .. note:: rather than inventing a formal .. deprecated:: version
   directive. I also didn't touch docs/user_guide/transformation/ LogCpTransformer.rst, which still describes it as fully independent —
   happy to update that in this PR or a follow-up, whichever you'd prefer.

The 0-handling-without-a-declared-constant question from the issue is left
for a follow-up, per the discussion.

Testing

pytest tests/test_transformation/ — 117 passed, including a new regression
test proving the C=0 default preserves the original fail-fast contract.