feat(assets): a bundle carries the images its sources declare (0.10.0)

Until now no reader in this package fetched, named, described or copied a
single image. `<img>`'s attributes were never read, a NISO-STS `<graphic>`
was walked past, a PDF was opened for its text alone, the converter's
markdown writer dropped every picture, and the only writer into a bundle
took `content: str`. The two lossiness warnings said so on every run, which
made the loss honest and did not make it smaller.

Measured on R761 Prosesskoden:2025, published as a 701-page PDF and as a
NISO-STS delivery: the process text is carried in full while 12 `Tabell N-N`
and 9 `Figur N-N` captions stand over nothing, because that publisher ships
those tables as raster pictures in both. Process 84's "toleranseklasse ...
er gitt i tabell 84-2" points at empty space.

THE GATE WAS WRITTEN FIRST AND RED. `tests/test_asset_gate.py` reads its
denominator out of the source (`page.images`, `word/media/`, `ppt/media/`,
`<img`, `<graphic`), never from a constant here. Measured at 332961a, built
from `git archive` and not from the editable tree: carried 0 of 8 local
images across 5 documents (9 declared), and no `assets/` at all. After: 8 of
8, with the ninth a remote source carried as a pointer without a file.

FIVE READERS PLACE, ONE MODULE DECIDES. `assets.py` owns what an image is
(sniffed from the bytes, never from the claimed extension), what it is
called (`<sha256[:12]>-<the source's own basename>`) and how it is pointed
at (one two-line block, one regex). `.xlsx` is deliberately not a row: a
block inside its pipe tables would break the `source_rows` locator, and 0 of
4 K2 workbooks hold media.

A PDF stream that is already a file is carried VERBATIM (29 of R761's 50
objects are DCTDecode); raw samples are encoded to PNG with stdlib zlib, so
no new dependency. Rendering the page region was the alternative and was
felled on determinism: a rasterised crop's bytes, and therefore the asset's
content-addressed name and the bundle's digest, would depend on the
installed rasteriser. What the encoder cannot express exactly is refused
with a code and counted, never approximated.

NO SIZE FLOOR, and that is a measurement: over the 4 828 image objects of
the K2 corpus the size distribution is a broad spread with no gap, unlike
OCR_CID_SHARE's bimodal one, so a threshold would be a number we chose.

ON BY DEFAULT, AND THE CONTROL IS TWO WHOLE BUILDS. The 43-document
reference corpus at 332961a versus rebuilt at HEAD with `--no-assets`:
865 files on both sides, `diff -rq` reports ONE difference, the added
`Images: NOT CARRIED` line in log.md. Every concept byte-identical.
Against the default: 453 -> 454 concepts, 865 -> 867 md, 0 -> 2 964 assets
(2 964 carried of 3 145 found, 4 622 pointers), 4.7 MB -> 115 MB, 2 414 s ->
3 088 s, peak RSS 6.26 -> 8.74 GB, 422 of 865 md files differ. The one new
concept has a measured cause: the pointers are body text, so a section
holding 146 of that document's images grew from 19.0 % to 30.6 % of the
extracted text and crossed `--outline-gate`'s 0.20 share clause.

THE IMAGE BYTES ARE NOT SCREENED. The guard is text-only, the pointer block
passes the gate as body text, the picture beside it passes nothing, and
log.md says so on every run.

Also fixed, both found by measuring rather than by reading:

- a markdown image is no longer read as a cross-reference. `structure._LINK`
  never looked at the character in front of the bracket, so every pointer
  would have arrived in the index as an edge to a concept that cannot exist.
- Door C carries the assets its merged concepts point at. Before this,
  importing a bundle built with `--assets` merged 6 of 6 concepts and wrote
  no `assets/` at all, so every pointer named a missing file.

Report: docs/2026-09-17-bilder-i-bundlen-trinn1.md
Spec proposal: docs/plan/okf-assets-section-6-4.md
Suite 1 955 passed / 1 skipped (from 1 896), ruff and mypy --strict clean.

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

tests/test_asset_structure.py

@@ -0,0 +1,55 @@
+"""An image is not a cross-reference, and a carried image is counted (0.10.0).
+
+Two door-level properties that the asset layer would otherwise get wrong in
+opposite directions.
+
+**A markdown image is not a pointer to another concept.** `structure._LINK`
+reads `[...](target)` and does not care what stands in front of the bracket, so
+every pointer this capability writes would arrive in the index as a `references`
+edge to a concept that cannot exist. That is the same defect K3-21 hit when the
+`Enclosing section:` line was read as body text and became a second, unresolved
+edge; the repair there was to append after derivation, and the repair here is
+that an image link is a different construct from a link.
+
+**A concept says how many images it carries.** A bundle that holds an image and
+says nothing about it is only half an improvement: the count is what lets a
+consumer, or `okf quality`, tell "this document had no figures" from "this
+build dropped them". The key is CONDITIONAL -- written only where the count is
+non-zero -- so a bundle of documents without images is byte-identical to one
+built before this existed.
+"""
+
+from __future__ import annotations
+
+from llm_ingestion_okf.structure import derive_document_structure
+
+POINTER = (
+    "# 84 Konstruksjoner\n\n"
+    "Toleranseklasse er gitt i tabell 84-2, jf. N500.\n\n"
+    "![Tabell 84-2](/assets/e54e5f5da0e8-tabell-84-2.png)\n"
+    "Image: graphics/tabell-84-2.png (120x90 px) -- Tabell 84-2\n\n"
+    "Se ellers [N100](n100.md).\n"
+)
+
+
+def test_an_image_pointer_is_not_a_reference() -> None:
+    structure = derive_document_structure(POINTER, source_file="r761.xml")
+    assert "/assets/e54e5f5da0e8-tabell-84-2.png" not in structure.references
+
+
+def test_the_links_beside_it_still_are() -> None:
+    """The known-positive in the same text: masking must not eat real edges."""
+    structure = derive_document_structure(POINTER, source_file="r761.xml")
+    assert "n100.md" in structure.references
+
+
+def test_the_asset_name_does_not_leak_a_number_into_references() -> None:
+    """A masked span is still masked: `84-2` inside the file name is not a subject.
+
+    The span is blanked rather than deleted for exactly this reason -- deleting
+    would move every later offset and break first-appearance order, and leaving
+    it would let the number scan read an identifier out of a file name.
+    """
+    structure = derive_document_structure(POINTER, source_file="r761.xml")
+    assert all("ASSETS" not in subject.upper() for subject in structure.references)
+    assert "N500" in structure.references