feat(design): give the canvas layout route a revision, so saves, reads and change events order themselves #3203

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Problem or opportunity

Follow-up from @egdev6's review of #3146 (Gentle Design canvas, layout and minimap; part of #2740).

assets/design-canvas/canvas-layout.js keeps the layout of a design consistent between the page and design/<slug>/layout.json with several counters and flags: saves in flight, reads in flight, change reads in flight, a pending read, layout views, and the move in progress. It recognizes the page's own save, when its file-change event comes back, by comparing JSON.stringify output with what it last saved.

That works, and each race has a regression test, but the rules are spread over several functions, and the string comparison depends on the server writing keys in the same order as the page sent them. Each new race so far needed another counter or flag.

Proposed outcome

The api/designs/<slug>/layout route returns a revision for the file (a counter or an ETag-like hash), on GET and in the POST reply:

• The page sends the revision it built on with each save, and the server can refuse a save based on a stale revision. That turns out-of-order saves and "another page changed it meanwhile" into one explicit answer.
• A read or a change event is applied only when its revision is newer than the one the page shows. That covers stale reads and the page's own save echo without comparing JSON strings.
• canvas-layout.js drops most of its counters in favor of that one rule.

Alternatives considered

• Keep the counters and flags, which are covered by tests today. That means less churn, but every new race adds state.
• Compare file modification times instead of a revision. That is coarser, and two saves within the same timestamp tick would look alike.

Additional context

• Review: feat(design-canvas): layout.json arrangements, moving artboards and the minimap #3146 (review by @egdev6, marked non-blocking for that PR).
• The same route already tells a missing layout.json from an unreadable one (422 unreadable-layout), so a revision fits next to that reply.