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>

README.md

@@ -308,6 +308,35 @@ in must not read like a bundle of documents that had none.
 text. The pointer block passes the gate like any other body line, and the file
 beside it passes nothing. `log.md` says so on every run that carries one.
 
+**Every carried image is one a model can be SHOWN.** A bundle that holds a
+picture nothing can read is worse than one that says the picture is missing:
+the count reports that it arrived. Measured over the frozen R761 delivery's own
+`assets/` (denominator 50): 29 JPEG, 2 PNG and **19 RLE8 BMP** — correct files
+that no model decodes. Every asset's type is read off its bytes and tested
+against the viewable set; a BMP is converted losslessly to PNG (8-bit
+uncompressed, 8-bit RLE8, 24-bit uncompressed), and anything else outside the
+set is refused with `asset_not_viewable` and a line in the concept saying what
+stood there. A BMP variant this reader does not express — RLE4, BITFIELDS,
+16- or 32-bit samples, a 12-byte BITMAPCOREHEADER — is `asset_bmp_unsupported`,
+a different fact about the document and a different thing to go and fix.
+
+The reader is stdlib and adds no dependency. Pillow, which this tree already
+carries transitively under `pdfplumber`, was measured first and rejected on two
+counts: images are carried on the CORE path, where `.html` and `.xml` need no
+`[extract]` extra, and an asset's name is its content digest — encoding through
+an installed library would make a bundle's identity move with that library's
+version, which is the property page rasterisation was felled over. Pillow is
+the independent decoder in the tests instead, and against it **19 of 19** of
+R761's real RLE8 assets convert with identical RGB, 2 366 365 pixels compared.
+
+A converted asset is ONE asset: one file in `assets/`, one pointer, one row in
+the accounting. The pointer's second line — where the source's own file name
+and the size in pixels already live — states the original media type, the
+original sha256 in full and the new one, so a reader can take the original
+delivery, run `shasum -a 256` and find the row.
+
+<!-- asset-viewable-media-types: image/gif,image/jpeg,image/png,image/webp -->
+
 **A size CEILING, read off the same corpora (0.10.1).** An image over
 `MAX_IMAGE_PIXELS` (40 000 000 pixels) or `MAX_IMAGE_BYTES` (256 MiB) is
 refused with `asset_too_large`, counted like every other refusal. The largest