feat: derive and set a document's schema_version

[wavekat-eason]

Why

The format grew a second version when book arrived, and the number leaked into the authoring surface: an editor that adds a book step to a v1 draft gets node "book" is a book step, which needs schema_version 2 (this document declares 1). The author placed a step; the answer named a number they have no way to know about.

Nothing here decides policy — these are the two primitives an authoring tool needs so it can keep the declared version in step with the components actually placed.

What

• requiredSchemaVersion(flow) (model.ts, next to KIND_MIN_SCHEMA_VERSION) — the lowest version that can carry a document's components. Floor of 1, because Math.max() of nothing is -Infinity and a new draft has no steps yet.
• setSchemaVersion(source, version) (mutate.ts, next to stampIdentity) — restates the declared version through withDoc, so comments, key order, and the ui block survive. Lowers as readily as it raises.
• required_schema_version(flow) — the Rust twin, in validate.rs beside kind_min_schema_version. setSchemaVersion gets none: mutate.rs doesn't exist because the daemon never edits documents, which that file already states.

Paired, they let an editor hold a draft at the lowest version that can run it — which is also the widest, since an engine only runs versions it knows. A document that needs nothing newer stays at 1 and stays runnable everywhere.

Not a migration

KIND_MIN_SCHEMA_VERSION's comment said "Documents are never rewritten", which reads as a contradiction sitting next to a function whose job is to rewrite the version. It isn't one — that promise is about stored documents, and it still holds. The comment now says so explicitly rather than leaving the next reader to work it out.

Tests

• New test/model.test.ts: v1-only → 1, book → 2, falls back when the component is removed, empty nodes → 1.
• test/mutate.test.ts: sets the version leaving comments and ui alone, lowers as well as raises, parse_failed on unparseable text.
• validate.rs: the three twin cases.

Both languages green — pnpm --filter @wavekat/flow-schema test (137) and cargo test -p wavekat-flow (53 + 3 conformance). cargo fmt --check and clippy --all-targets clean. Corpus untouched: this adds no schema change and no case.

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https://claude.ai/code/session_01AFE4mZNdkfPUBbTTwW4e7A