Credibility pass: comments state constraints, not development history

A skeptical-cloner review flagged the patterns that read as AI-iteration
residue: test comments and section headers narrating the review process
that produced them, "legacy format" framing in a days-old repo, shadow
re-imports appended without reading file headers, one genuine machine
leftover (FIXTURE_CACHE = FIXTURE_CACHE), and a few register slips.
Every history-narrating comment is rewritten as the timeless invariant
it was guarding, test sections are grouped by behavior, function-local
imports are hoisted, and the README loses its one marketing clause and
heaviest dash runs. No behavior changes; 176 tests unchanged and green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_models.py

@@ -119,20 +119,12 @@ def test_error_document_raises():
 
 
 def test_search_all_items_malformed_raises():
-    import pytest
-
-    from bggpipe.models import BGGResponseError, parse_search
-
     xml = '<items total="2"><item type="boardgame"/><item type="boardgame"/></items>'
     with pytest.raises(BGGResponseError):
         parse_search(xml)
 
 
 def test_search_partial_malformed_tolerated_with_warning():
-    import pytest
-
-    from bggpipe.models import parse_search
-
     xml = (
         '<items total="2">'
         '<item type="boardgame" id="13"><name type="primary" value="CATAN"/></item>'