docs: the PDF path reached the ADR 0020 stages too

[willhea]

Two documentation statements went false when #639 and #638 merged six minutes apart.

#639 separated the PDF pipeline into the same four ADR 0020 stages and landed first; #638 landed second still saying the PDF path was unmigrated. Nothing could have caught this: #639 touches neither docs/decisions/0020-matching-stages.md nor docs/architecture.md, so there was no textual conflict, no failing test, and no merge-queue signal — only prose that had quietly stopped being true.

What changed

docs/decisions/0020-matching-stages.md — the Let classification re-consult similarity rejection cited diff_pdf._hunk_for_paired_blocks as still thresholding a correspondence score inside classification, and used that as the reason the record does not unify the two implementations.

Slice 6a closed precisely that violation. The function no longer receives a similarity at all; the moved-vs-modified rule moved unchanged into pdf_round1_move_basis, an assignment stage, and what remains is the modified emitter. The entry now says both pipelines hold the separation, and keeps the half that is still true — they share the boundary rule, not an implementation.

docs/architecture.md — said the PDF path was not migrated. It now says what it is: the same four stages, reached separately, with its own byte-identity gate (tests/test_pdf_canonical_baseline.py) and its own boundary tests. It also states why the implementations stay apart, since that is the question a reader asks next: an XML observation is a parsed tree node and a PDF observation is a reconstructed block, so a shared matcher would have to pick one representation and lose the other.

Verification

Every symbol was checked against the merged tree, not against #639's description:

• pdf_round1_move_basis — src/deltatrack/diff_pdf.py:704
• tests/test_pdf_canonical_baseline.py — exists
• PdfObservation addresses a _Block, mirroring diff_bill.Observation → BillNode

A sweep for other "PDF is not migrated" claims found none. The remaining PDF-path statements concern lossiness and pre-publication availability, which are unaffected.

Documentation only — git diff over src/ and tests/ is empty. Docs and ADR-index gates pass (34); ruff check and ruff format --check clean.

Refs: ADR 0020

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