feat(studio): flow designer authoring for the approval revise loop (ADR-0044)

[os-zhuang]

Follow-up to #1744 / PR #1769 (ADR-0044), split out on purpose.

The engine, service, REST and Console inbox for send-back-for-revision are merged: an approval node may declare a revise out-edge to a wait node, with a type: 'back' edge closing the loop back into the approval node (cycle validation now runs on the graph minus declared back-edges), guarded by maxRevisions on the node config.

What's missing is authoring: today the revise loop is reachable only by hand-editing flow JSON (see showcase_budget_approval in examples/app-showcase/src/flows/index.ts for the canonical shape). The Studio flow designer should:

1. offer the revise branch on the approval node's out-edge labels (alongside approve/reject), with maxRevisions already surfaced via the node's published configSchema;
2. let the author draw/mark a back-edge (type: 'back') and render it distinctly (dashed/curved return arrow);
3. validate the reduced-graph DAG rule client-side so an unmarked cycle is flagged while a declared revise loop passes;
4. ideally a one-click "add revision loop" affordance that drops the wait node + both edges in one gesture.

References: docs/adr/0044-approval-send-back-for-revision.md, APPROVAL_BRANCH_LABELS (packages/spec/src/automation/approval.zod.ts), FlowEdgeSchema.type (packages/spec/src/automation/flow.zod.ts).

🤖 Generated with Claude Code

[os-zhuang]

Designer authoring (this issue's scope) shipped in objectui — all four authoring gaps done and merged:

• feat(studio): author the approval revise loop in the flow designer (ADR-0044) objectui#1954 — revise branch picker + back-edge authoring (dashed return arc, excluded from auto-layout) + client-side DAG-modulo-back-edges validation + one-click "add revision loop"
• feat(studio): simulate approval decisions in the flow debugger objectui#1955 — the Debug simulator now walks the loop (approval suspends; approve/reject/revise decision buttons)
• feat(studio): surface flow validation errors inline on the canvas objectui#1958 — un-declared cycles surfaced inline on the canvas (red edges/nodes + error banner)

Remaining AI Build-mode awareness is tracked in #2274, whose compile-time guardrail slice shipped in #2279. Closing — the flow-designer authoring scope is complete.