Fixes #1194

PR Summary — Fix _remove_circular_references nulling shared non-circular config values (Issue #1194)

What changed

pdf2zh/config.py — ConfigManager._remove_circular_references:

• The seen set is now scoped to the current recursion (ancestor) path instead of one
  never-shrunk global set for the whole traversal. Container ids are added before
  recursing into children and discarded (seen.discard) in a finally block when
  recursion unwinds, so a diamond (shared but non-circular) reference is preserved
  while a true ancestor-path cycle still collapses to None.
• Tracking now applies only to dict and list containers. Previously every object
  (including immutable scalars like small ints/strings, which share identity via
  interning) was added to seen, so e.g. [1, 1] would have serialized as
  [1, None]. Scalars are now returned verbatim.

Why it addresses the issue

The issue's repro returned {'x': {'a': 1}, 'y': None} for a shared dict referenced
from two sibling keys (and [[1, 2], None] for a shared list), causing silent data
loss on _save_config. After the fix, both cases round-trip fully, while a genuine
self-referential dict (d["self"] = d) still collapses to {"self": None} as
intended, and nested cycles (e.g. a self-referential list inside a dict) still
collapse at the exact cyclic edge.

Verification

• Reproduction script covering 5 cases (dict diamond, list diamond, true dict cycle,
  repeated immutable scalars, nested list cycle): all assertions passed.
  Command: python3 _repro_1194.py (temporary script, removed after the run).
  Result: ALL REPRO CHECKS PASSED.
• Syntax check: python3 -m py_compile pdf2zh/config.py — passed with no output.
• The project's full suite (pytest . per .github/workflows/python-test.yml) could
  not be run here: neither pytest nor uv is installed in this environment, and the
  suite additionally requires system dependencies (libreoffice) and network-cached
  BabelDOC assets. There are no existing tests covering ConfigManager, so no
  narrower test target exists.