feat(extract): implement pdf behind the [extract] extra with pdfplumber

Order G2a. Populates the optional `[extract]` extra for the first time with
one parser, `pdfplumber>=0.11.10,<0.12` (MIT), and wires `pdf` through it.
The default install is untouched: exactly one runtime dependency, stdlib
otherwise, enforced by test_packaging.py.

The gate for `pdf` becomes an import probe rather than a frozenset membership
test, exactly as extract.py's docstring had promised. The rejection does not
change: without the extra, `pdf` still raises `extractor_extra_missing` with
the same message. That behaviour is asserted UNCONDITIONALLY via a sys.modules
monkeypatch, so it holds on machines where the parser is installed too — a
skip would have preserved nothing there. Verified in a clean venv without the
extra: 589 passed, 7 skipped; with it, 596 passed.

`docx`/`xlsx` are unchanged and still fail fast — the extra names exactly what
it ships.

The parser choice was forced by measurement, not preference (b73dd9d,
docs/2026-08-21-g2-pdf-extraction-measurement.md): on a real requirement table
pdfplumber keeps 4 of 4 rows with label and value on one line, where pypdf,
pdfminer.six and pymupdf each keep 0 of 4. pymupdf is additionally out on
licence (AGPL-3.0), which an MIT package must not push onto a consumer.

Three facts from that measurement are now carried in code rather than in a
report:

- Extracted text is pinned to an exact transitive parser version
  (pdfplumber pins pdfminer.six==20260107; date-stamped, no stability
  contract). tests/test_extract.py freezes the expected text of a committed
  hand-written fixture so a parser upgrade breaks something visible instead of
  drifting silently. Reasoning at the declaration site and in
  tests/fixtures/README.md.
- Determinism within a version is now held by a test, not only measured once.
- Drawn content does not survive extraction. Every pdf extraction emits the
  new `ExtractionWarning`: figures have no text to recover, so a bundle built
  from drawn documents is incomplete by construction. Stated categorically
  rather than detected — deciding "is there a figure here" is the layout
  heuristic G2b declined.

Two new error codes, both mirroring existing patterns: `extractor_empty_pdf`
(a scanned/image-only PDF, refused rather than persisted as an empty concept)
and `extractor_pdf_error` (parser failure wrapped, never leaked).

Structured table recovery (G2b) is NOT implemented and is documented as out of
scope: two independent parsers return the same wrong shape, so the breakage is
document geometry, not a library choice. PDFs enter as prose.

Also corrects an install promise this change would otherwise have published:
the README no longer presents a bare `pip install 'llm-ingestion-okf[extract]'`
as working, because the package is not on an index.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HtNhsdHnMGtMi7U2mvMU8z

pyproject.toml

@@ -25,9 +25,33 @@ classifiers = [
 dependencies = ["llm-ingestion-guard>=0.3,<0.4"]
 
 [project.optional-dependencies]
-# Reserved for binary file-type extraction parsers (pdf/docx/xlsx).
-# Populated when Door B's binary extraction is implemented.
-extract = []
+# Binary file-type extraction parsers. OPT-IN ONLY: this extra pulls binary
+# wheels (pillow, pypdfium2) and a transitive tree that core must never have —
+# the "exactly one runtime dependency" rule above covers the default install,
+# and this extra is outside it by construction.
+#
+# `pdf` only. `docx`/`xlsx` remain fail-fast: the extra names the parsers it
+# actually ships, so a consumer installing it gets what the error message
+# promised and nothing else.
+#
+# WHY pdfplumber, and why the floor is not free (measured 2026-08-21,
+# docs/2026-08-21-g2-pdf-extraction-measurement.md): on a real Vegnormalene
+# requirement table pdfplumber keeps 4 of 4 rows with label and value on the
+# same line; pypdf, pdfminer.six and pymupdf each keep 0 of 4, emitting all
+# labels then all values, which a downstream reader can only re-pair by
+# guessing. In a `krav` document that is a wrong answer that looks right.
+# pymupdf is additionally out on LICENSE (AGPL-3.0 or commercial) — this
+# package is MIT and an extra must not hand a consumer copyleft they did not
+# choose.
+#
+# PARSER VERSION IS PART OF THE OUTPUT CONTRACT. pdfplumber pins
+# `pdfminer.six==20260107` exactly, and pdfminer.six ships date-stamped
+# releases with no stability contract. Extraction is deterministic WITHIN a
+# parser version (measured, 5 configurations) and NOT guaranteed across one.
+# `tests/test_extract.py` holds that promise against a committed fixture, so
+# widening this range makes a test go red instead of letting extracted text
+# drift silently. See tests/fixtures/README.md.
+extract = ["pdfplumber>=0.11.10,<0.12"]
 
 [dependency-groups]
 dev = ["pytest>=8", "mypy>=1.14", "ruff>=0.9"]