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>

README.md

@@ -309,6 +309,36 @@ this package chose, and it would silently drop somebody's small table.
 
 <!-- cli-default-assets: on -->
 
+`--accounting PATH` accounts for the CONTENT, not only the files. Before
+extraction, every source document is inventoried in a per-format element
+vocabulary: headings, paragraphs, tables, cells, images, pages, and so on.
+After the run, each element gets exactly one fate:
+
+- **carried:** all of its text is in the concepts written for the document,
+  or its image is in `assets/`;
+- **pointer:** a remote image, which is never fetched, or a markdown image
+  reference kept verbatim;
+- **a coded rejection:** the gate's or the reader's code.
+
+The result goes to PATH as JSON and into `log.md`. The build exits 1 when an
+element has no fate or has two. A document the gate refuses is logged as
+`<file>: <M> elements found in the source, 0 carried: document rejected
+`<code>``, and the `Images` bullet then counts what the sources declare.
+
+"Carried" means the text is present, not that it is in the right place. A
+short element such as a section label can be found elsewhere in the same
+document. The judge is `tools/okf_accounting_gate.py`, which compares the
+inventory against an independent witness.
+
+Two things hold with or without the flag:
+
+- `okf build` exits 1 when it extracted at least one document and persisted
+  none.
+- An image file beside a document is counted once. If a persisted document
+  carried it, it is in the conservation identity's own column
+  (`merged + files carried through a document + coded rejections = N`);
+  otherwise it is a coded rejection.
+
 `--frontmatter KEY=VALUE` (repeatable) stamps a key on every concept of the
 run, for what the operator knows and the document does not say — an edition,
 a publisher's address. It splits on the first `=` and writes the value