feat(errors): register the four converter codes

Registered before any step raises them, so no later step invents a spelling:

- `extractor_binary_missing`   -- the converter binary is absent (distinct
                                  from the extra not being installed)
- `extractor_binary_version`   -- present, but not the pinned version
- `extractor_convert_error`    -- the converter failed on this file
- `extractor_empty_conversion` -- the converter returned no text

Also widened the `extractor_extra_missing` type list, which still read
"pdf/docx/xlsx".

Denominators recounted after the change rather than carried from a note --
the stale 49/48 figure is what made the recount a step requirement:

  code bullets in errors.py        50 -> 54
  distinct codes                   49 -> 53   (one code documented twice)
  test definitions in this suite   56 -> 57   (one parametrized definition,
                                               four cases -- one per code)

The four tests assert only what is true at this step: the code is documented
and an ExtractionError carries it. The resolver and the seam replace each with
a behavioural raise-site test; a code that never gains one stays visible here
as a test that still only reads a docstring.

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

tests/test_error_codes.py

@@ -356,6 +356,30 @@ def test_extractor_extra_missing(monkeypatch: pytest.MonkeyPatch) -> None:
     assert code_of(excinfo) == "extractor_extra_missing"
 
 
+# The four converter codes are registered here BEFORE anything raises them, so
+# no later step invents a spelling. Until the converter lands (the resolver in
+# `_pandoc.py`, then the seam in `extract.py`), the falsifiable claim is exactly
+# this: the code is documented in the registry and an `ExtractionError` carries
+# it unchanged. Those steps replace each of these with a behavioural raise-site
+# test; a code that never gains one would show up here as a test that still
+# only reads a docstring.
+
+
+@pytest.mark.parametrize(
+    "code",
+    [
+        "extractor_binary_missing",
+        "extractor_binary_version",
+        "extractor_convert_error",
+        "extractor_empty_conversion",
+    ],
+)
+def test_converter_code_is_registered_and_carried(code: str) -> None:
+    registry = ExtractionError.__doc__ or ""
+    assert f"`{code}`" in registry, f"{code} is not documented in the registry"
+    assert ExtractionError("x", code=code).code == code
+
+
 @requires_extract
 def test_extractor_empty_pdf() -> None:
     with pytest.raises(ExtractionError) as excinfo: