Visual audit contract cannot disposition text a native GT rasterizes #1866

Problem

scripts/check_visual_pr.py can disposition a layout finding with ref:<id> only
when it is an exact painted-text-visibility occurrence
(scripts/check_visual_pr.py:814). A missing/extra text finding always needs an
open issue, even when a native Office export proves the GT simply cannot paint
that text.

The case that has no expressible disposition today is a native GT that
rasterizes text we paint as selectable text. On pages 13-14 of the #1220 deck
(GENERAL SERVICES.pptx, SHA-256
17924ec3b27646a2c1b2bb711b845360b713cc7052b3cdde9df752f9ceded3a9) the GT is
the native PowerPoint 16.113.1 export, which draws the slide-number field as a
23.28 x 21.12pt clip_image_mask at (907.92, 491.52). compare_layout.py
therefore reports our 13 / 14 as extra text with no GT counterpart. #1667
owns that finding and has to stay open forever as its citation.

Two independent gates block a manifest here, both verified by running
validate_reference_difference_document on a native-GT document:

reference_export.application must identify the non-native GT exporter
difference 1 kind must be painted-text-visibility or render-clusters

• scripts/reference_exporter_differences.py:59 rejects a reference_export
  naming a native Office application, so a comparison whose GT is the native
  export cannot state its own provenance.
• scripts/reference_exporter_differences.py:238 accepts only
  painted-text-visibility and render-clusters kinds.
• scripts/check_visual_pr.py:658 additionally requires a nonzero decoded-pixel
  delta between gt.jpg and native.jpg, which are the same render when the GT
  is the native export.

So the manifest shape assumes a third-party reference sitting beside a native
export. A native-GT comparison has no third party, and "reference-side" is a
misnomer for it — the difference is GT-side, where GT is already the best
available truth.

This is the missing/extra-text sibling of #1840, which asks for the same thing
for shift findings.

Acceptance

• Add a difference kind that records a text line one exporter rasterizes: exact
  page, label, side (gt or out), and occurrence, matched against the current
  report's lines.missing_text / lines.extra_text the way
  _reference_visibility_key matches a visibility mismatch.
• Add a provenance form for a comparison whose GT is the native export, so the
  document can omit or mark the absent non-native reference instead of forging
  one, and so the gt.jpg / native.jpg pixel-delta gate does not demand a
  difference between two renders of the same PDF.
• Let Layout audit text flow accept ref:<id> only for an exact matching
  record; keep every other missing/extra/wrap/reflow finding on an open issue,
  and keep page-, region- and label-wide approvals rejected.
• Cover the new path in scripts/tests/test_check_visual_pr.py and document it
  in assets/bugfixes/README.md.

Related: #1667, #1840, #1220