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

src/llm_ingestion_okf/errors.py

@@ -86,6 +86,23 @@ class ExtractionError(IngestError):
       xlsx) was given but the optional extra is not installed
     - `extractor_decode_error` — a text-type file's bytes are not valid UTF-8
     - `extractor_empty_csv` — a CSV has no header row
+    - `extractor_empty_pdf` — a PDF yielded no text on any page (a scanned or
+      image-only document); refused rather than persisted as an empty concept,
+      which would be the silent skip this registry exists to prevent
+    - `extractor_pdf_error` — the PDF parser failed on the file's bytes; the
+      third-party exception is wrapped, never leaked
+    """
+
+
+class ExtractionWarning(UserWarning):
+    """Extraction succeeded, but the output is lossy in a way worth stating.
+
+    Text extraction recovers text. Anything a PDF *draws* — figures, diagrams,
+    images — has no text to recover, so a bundle built from drawn documents is
+    incomplete by construction. That is categorically true rather than
+    document-specific, so it is warned about rather than detected: deciding
+    "is there a figure on this page" is a layout heuristic this library does
+    not own. A named class so a consumer can filter it deliberately.
     """