changelog.d/5149-docs-schema-6-canvas.md

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- **Docs content schema 6 admits the Canvas surface (#5149).** Block placements, notes, shapes and connectors live in their own `surface:*` roots beside the page; a move changes only geometry, so page text, block hashes, Markdown and comment anchors are identical to schema 5. Schema 6 keeps every capability schema 5 released: agents propose on Page blocks of a Canvas document and a person accepts or rejects, with formatting and canvas geometry kept. A migration widens the closed schema checks on propose mandates and proposals to schema 6; existing rows are kept unchanged. Schemas 1-5 keep refusing the surface and load, validate and hash unchanged; new documents are still created at schema 5, and a document becomes schema 6 only by an explicit `"schema": 6` initialize or the human-only upgrade. From schema 5 on, a restore never lowers a document's schema. The service validates the merged state on every save, and the surface has its own 512 KiB history share: a save past it is refused with 413 `surface_quota`. An editor that does not know schema 6 opens such a document read-only.