test(getcwd): lock in KindNull enter + exit-time cwd resolution

Audit of the getcwd(2) tracing path. getcwd's args[0] is a char *buf
OUTPUT buffer: the kernel writes the absolute cwd path into it and the
contents are only valid AFTER the syscall returns. Reading it at enter
would capture an empty/garbage string, so getcwd is correctly KindNull
at enter and the cwd is resolved at EXIT from /proc/<tid>/cwd when the
return value is positive (handleNullExit). Family FS, docs and drift
tests already aligned; no behavior change required.

Add lock-in tests pinning the correct behavior:
- generate: strengthen TestClassifyNullGetcwd to assert the enter kind
  is never KindPathname/KindName and no pathname field is captured;
  add TestClassifyByFieldGetcwdBufNotPath proving the generic field
  classifier never treats char *buf as a path (defense-in-depth).
- eventloop: add GetcwdFailureEventTest asserting that a failed getcwd
  (negative errno, e.g. -ERANGE) attaches no cwd path, and document the
  output-buffer nuance in the success-case test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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