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

CLAUDE.md

@@ -17,7 +17,13 @@ one boundary rule:
   file-type→text extraction lives HERE (the guard is text-only). v1 core:
   `md`, `txt`, `csv`, `json`, `html` (stdlib). `pdf`/`docx`/`xlsx` only via
   the optional `[extract]` extra; without it those types are rejected
-  fail-fast.
+  fail-fast. The extra ships `pdfplumber` for `pdf` (chosen on ONE measured
+  property: it keeps a requirement table's label and value on the same line
+  where three alternatives do not); `docx`/`xlsx` still ship no parser.
+  Structured table recovery is **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, and drawn content (figures) does not
+  survive extraction at all, which every `pdf` extraction warns about.
 - **Door C — external bundle import:** third-party OKF bundles are assessed
   per concept via the guard's `okf.import_bundle`; only concepts clearing the
   guard's non-blocking floor are merged/indexed here. Two invariants, both
@@ -176,7 +182,11 @@ stdlib, and a packaging test enforces it. Only `guard_adapter.py` imports the
 guard; importing the package does not. Install channel until the package
 index exists (a direct reference is a channel, not the pin):
 `pip install "llm-ingestion-guard @ git+https://git.fromaitochitta.com/open/llm-ingestion-pipeline-security.git@v0.3.4"`.
-Binary extraction parsers live behind the `[extract]` extra only.
+Binary extraction parsers live behind the `[extract]` extra only — today
+`pdfplumber>=0.11.10,<0.12` for `pdf`. Extracted PDF text is pinned to an
+exact transitive parser version (`pdfminer.six==20260107`), so widening that
+range is a fixture migration, guarded by a frozen literal in
+`tests/test_extract.py`; see `tests/fixtures/README.md`.
 
 Phase 4 adds a `node/` half: Node/ESM with zero npm dependencies
 (`node:` builtins only), both importable and CLI-invokable, consumed by