feat(assets): every carried image is one a model can be shown

Chosen: a stdlib BMP reader, because `read_image` is on the CORE path and an
asset's name is its content digest. Measured first, as the order requires:
Pillow 12.3.0 IS in this tree (transitively under `pdfplumber`) and it DOES
decode RLE8 correctly -- a hand-written stdlib decoder and Pillow agree on
19 of 19 of R761's real files, RGB per pixel. So the choice does not rest on
capability. It rests on two properties of this package: `.html` and `.xml`
carry images with no `[extract]` extra installed, so a Pillow converter
either makes a core path depend on an optional binary wheel or buys the
second runtime dependency; and encoding through an installed library would
make a bundle's identity move with that library's version, which is the
property 0.10.0 felled page rasterisation over and `encode_png`'s docstring
already defends. Pillow keeps the job it is good for: the INDEPENDENT decoder
in the tests, on neither side of the conversion.

The defect, measured over the frozen R761 delivery's `assets/`, denominator
50: 29 JPEG, 2 PNG and 19 RLE8 BMP. The 19 are byte-correct files nothing
reads, so 19 figures were present and invisible while `images: N` reported
that they had arrived.

- `VIEWABLE_MEDIA_TYPES` is tested against every asset's SNIFFED type, so it
  is a property and not a list of formats we met. WebP is on it and `sniff`
  does not recognise one; the limit is stated, not implied.
- `bmp_to_png`: 8-bit uncompressed, 8-bit RLE8, 24-bit uncompressed. All five
  RLE8 opcodes. 19 of 19 real files convert with RGB identical to Pillow's
  decoding of the source, 2 366 365 pixels compared.
- `asset_not_viewable` and `asset_bmp_unsupported`, both published, both
  leaving the concept's "not carried" line.
- Traceability on the pointer's second line, where the rest of the asset
  metadata already lives: original media type, original sha256 in full, new
  sha256 in full. A converted asset is ONE asset.
- The ceiling is paid on the DECLARATION before a row is allocated, and an
  RLE run is one clipped slice -- painting pixel by pixel leaves the memory
  bounded and the CPU unbounded.

Two repairs the change forced, each measured rather than assumed:

- `tests/test_assets.py`'s "dimensions absent is absent" used a TIFF, which
  is now refused before `read_image` returns. The property still has a
  reachable case -- a JPEG whose frame header never arrives -- and uses it.
- `asset_holds` in the accounting gate proved a carry by hashing the SOURCE
  file, which a converted image's bundle cannot satisfy. It now also reads
  the two digests the bundle states and HASHES THE ASSET ITSELF, so a bundle
  claiming a conversion it did not perform still fails.

`tools/okf_asset_census.py` is the committed instrument for the
known-positive: one row per image, from two pinned trees. It was caught by
the rule it serves -- its first version handed `_pdf_images` the wrong page
object and reported 0 images over 67 PDFs with exit 0. The attribute is
asserted now and a known-positive runs before the sweep.

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

src/llm_ingestion_okf/errors.py

@@ -151,6 +151,20 @@ class ExtractionError(IngestError):
       statistic about the first untrue. Refused before the stream is read: a
       negative dimension multiplies to a negative pixel count, under which
       every bound reads as satisfied
+    - `asset_not_viewable` — the bytes are a real image in a format no model
+      can be SHOWN (TIFF, JPEG 2000), and this package has no lossless
+      conversion for it. DISTINCT from `asset_type_unknown`, which says the
+      bytes are not an image at all: this one says they are, and carrying them
+      would put a file in the bundle that the `images: N` count reports as
+      arrived and nothing downstream can read. Measured 2026-09-19 on the
+      frozen R761 delivery: 19 of its 50 assets were carried in exactly that
+      condition, as RLE8 BMP
+    - `asset_bmp_unsupported` — a BMP variant this reader does not express
+      (RLE4, BITFIELDS, 16- or 32-bit samples, a 12-byte BITMAPCOREHEADER, a
+      palette over 256 entries). DISTINCT from `asset_not_viewable`, which
+      says there is no conversion route for the format at all: this one says
+      there is one and this file is outside it, which is a different fact
+      about the document and a different thing to go and fix
     - `asset_pdf_unbounded` — the image is reached through a PDF stream filter
       this package has no measured cost ratio for (`LZWDecode`,
       `RunLengthDecode`, `CCITTFaxDecode`, `/Crypt`, anything unknown), or