Hide private fields in `Struct.pformat()` output

Skip fields starting with `_` in pretty-printed struct output
to avoid cluttering displays with internal/private state (and/or accessing
private properties which have errors Bp).

Deats,
- add `if k[0] == '_': continue` check to skip private fields
- change nested `if isinstance(v, Struct)` to `elif` since we
  now have early-continue for private fields
- mv `else:` comment to clarify it handles top-level fields
- fix indentation of `yield` statement to only output
  non-private, non-nested fields

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/msg/pretty_struct.py

@@ -126,13 +126,17 @@ def iter_struct_ppfmt_lines(
             str(ft)
         ).replace(' ', '')
 
+        if k[0] == '_':
+            continue
+
         # recurse to get sub-struct's `.pformat()` output Bo
-        if isinstance(v, Struct):
+        elif isinstance(v, Struct):
             yield from iter_struct_ppfmt_lines(
                 struct=v,
                 field_indent=field_indent+field_indent,
             )
-        else:
+
+        else:  # top-level field
             val_str: str = repr(v)
 
             # XXX LOL, below just seems to be f#$%in causing