test(extract): the file that IS the product had no reader, so state what one owes

RED, 11 failing. Reproduced first, before any code: `okf build` on the folder
a publisher's own viewer delivers gives **110 of 110 unreadable, 0 plans, exit
2**, with `no extractor is registered for file extension '.xml'`. The
conservation identity `merged + coded rejections == N` is never written at all
-- the run aborts earlier on `FAILED - no segmentation plans` -- so the hole
was not even visible as a count.

The one xml file IS the whole product: R761 Prosesskoden:2025, the document
round 12 met as a 701-page PDF, in NISO-STS form. Measured on the file with
`xml.etree.ElementTree`: 7 715 `<sec>`, 2 761 with a `<title>`, 4 954 with a
`<label>` and no title, 10 `<table-wrap>`, 12 528 `<p>`, root `<standard>`, 0
`<!DOCTYPE` and 0 `<!ENTITY`. Its `<sec>`-nesting depths over the titled
sections are 28/118/500/1141/868/97/9 -- row for row the fasit's own
distribution. The ceiling is therefore structural rather than computed.

FOUR HAND-WRITTEN FIXTURES, none through `make_fixtures.py` and none
serialised by `ElementTree`: a library that writes and reads its own format
proves only that it agrees with itself. A known-positive STS mini, generic
non-STS xml, a `<!DOCTYPE` with an entity expansion, and a malformed file.

Two assertions that already existed are extended rather than duplicated: the
converter fence, because a file routed to the converter is read by a second
parser that never sees this reader's DTD refusal, and the evidence table,
because a row without a class is the failure that test exists for.

pytest -q: 11 failed, 1554 passed, 1 skipped.

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

tests/fixtures/README.md

@@ -20,6 +20,22 @@ and reproducible from that one file.
 | `outline-collision.pdf` | **Two bookmarks whose destinations resolve to the same line** — the tree's root node and a front-matter node, both on line 0, which is the shape R761 carries. The marks are collected in a dict keyed on the line index, so without this fixture the second node is dropped with nothing counting it: 2 763 nodes in, 2 762 marks out, `unresolved` at 0. A one-bookmark-per-line fixture cannot see that. |
 | `three-page-krav.pdf` | Three pages, one line of text each, and **the middle page carries no text operators**. The extractor drops empty pages, so the last page's text belongs to page 3 — which is what separates a page NUMBER from a count of the pages that produced text. Two pages could not tell those apart. |
 
+## The XML fixtures
+
+Hand-written text, directly in this directory — **not** through
+`make_fixtures.py`, which builds binary containers, and never serialised by
+`ElementTree`. The reason is the same one the OOXML policy states: a library
+that writes and then reads its own format proves only that it agrees with
+itself, and an `ElementTree`-written fixture would stay green through any
+round-trip-symmetric defect.
+
+| Fixture | What it is for |
+|---|---|
+| `sts-mini.xml` | The known-positive. A `<standard>` root with `<sec>` at three nesting levels carrying `<label>`+`<title>`, two lettered points (`a)`, `b)`) with a **label and no title**, one `<table-wrap>` with a label and two rows, `<p>` bodies, a `<list>`, and **one unnumbered section** (`Forord`, `<title>` with no `<label>`) mirroring the single such section in R761. The lettered points are what the 4 954 label-only `<sec>` in that document look like: promoted to headings they would bury its own 2 761. |
+| `generic-feed.xml` | Known-negative: XML that is **not** STS. It must produce text and ONE plan — never zero, never a crash, and never element names promoted to headings. |
+| `xml-doctype-bomb.xml` | Known-negative, security: a `<!DOCTYPE` with a small nested-entity expansion. It must be refused by `code`, and the test asserts the expansion appears in **no** output, including the error text. Small on purpose — the point is that it is never parsed, not that it detonates. |
+| `xml-malformed.xml` | Known-negative: an unterminated tag must raise a typed `ExtractionError`, not leak `ParseError` and not yield zero concepts in silence. |
+
 ## The office fixtures
 
 `two-line-krav.docx`, `no-styles-krav.docx` and `two-line-krav.xlsx` are