20260901T1917Z fold stages doc into architecture

What. Deleted docs/design/stages.md and folded three parts of it into architecture.md: the projections model (renderings and chunks by parser_version, claim_values by normalizer_version, plus the three rules that keep rebuilds safe), extraction_run keying by (event revision, ontology version, prompt version, model) with the domain-row projection in the same transaction, and the split between canonicalization (v1) and search-form expansion (matching, PRD milestone 5). The rest of the doc, its C1-C5 boundary criteria and the three-stage runtime split, is dropped as not correct. PRD section 7 and D15 no longer name ingest/summarize/extract; they speak of pipeline phases.

Why. Nacho removed the doc and the built stage packages in 550ac48 to clear the slate for a stage decomposition Agus defines; per Agus (2026-09-01) only the three items above were correct, and one design doc is simpler than two. Rule 2 of the projection rules is the load-bearing one: it is what stops a normalizer_version bump from triggering a full re-extraction.

Verify. rg -n "stages\.md" docs/ changelog/ returns nothing. The Projections section of architecture.md carries the table and the three rules.