feat(accounting): okf build accounts for every source element

okf build --accounting PATH inventories every source document before
extraction, in the gate's per-format vocabulary, and after the run gives
each element one fate (carried / pointer / coded rejection), written as
JSON and summarised in log.md. "carried" is checked against the written
concept bodies, so a gate that drops a line is found (test). Exit 1 on
anything unaccounted or double-booked. Opt-in: +744 s (+19 %) on the
43-document reference corpus, and that corpus fails the check on 24 real
losses (22 images on text-less PDF pages, 2 docx Title paragraphs).

Changed without the flag:
- okf build exits 1 when it extracted documents and persisted none.
  Door B and corpus.measure are unchanged. One test relied on exit 0.
- An image file carried through a persisted document is its own K1b
  column, no longer also extractor_unknown. The set is what the resolver
  actually carried (ExtractedDocument.files), never a byte match.

tools/okf_accounting_gate.py (checks untouched) is green on all six rows,
R761 110 of 110 under both gates.

Report: docs/2026-09-17-innholdsregnskapet-bygget.md

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

tests/test_cli_gate.py

@@ -106,7 +106,9 @@ def test_build_refuses_a_document_the_real_guard_refuses(tmp_path: Path) -> None
     pytest.importorskip("llm_ingestion_guard")
     inbox = _inbox(tmp_path, {"carrier.md": CARRIER})
     bundle = tmp_path / "bundle"
-    assert _build(inbox, bundle) == 0
+    # 1, not 0, since the content accounting: a build that extracted a document
+    # and persisted none is not a success, however every refusal was coded.
+    assert _build(inbox, bundle) == 1
     assert _concepts(bundle) == [], "a document the guard refuses must not reach the bundle"