diagram: a failed flow-attach patch silently discards the drawn flow

[bpowers]

Problem

In src/diagram/Editor.tsx, handleFlowAttach early-returns when applyPatchOrReportError fails, without updating the view. By that point Canvas has already cleared its in-creation staging state on pointer-up, so the flow the user just drew simply vanishes -- the only feedback is a transient error toast.

Why it matters

This is silent loss of user work. Observed live during the June 2026 flow-editing audit against a corrupted project where the attach patch failed repeatedly: every drag-create gesture produced nothing visible beyond a toast, making the editor appear broken with no path forward for the user. Even in less pathological cases, a single failed patch throws away a completed gesture.

Component

• src/diagram/Editor.tsx (handleFlowAttach)
• src/diagram/drawing/Canvas.tsx (in-creation staging cleared on pointer-up before the patch outcome is known)

Possible approaches

• Re-stage the gesture on patch failure so the drawn flow remains in its in-creation state and the user can retry or adjust.
• Keep the drawn geometry on the canvas with an error badge/annotation instead of dropping it.
• A more persistent error surface than the transient toast (e.g. a dismissable banner explaining why the attach failed).

Context

Identified during the June 2026 flow-editing audit; listed in the "Deferred" section of docs/dev/flow-editing-audit-2026-06.md but had no tracking issue until now.