fix(spec): fallbackNodeId 墓碑指向真正路由 fault 的键 (#6094)

`flow.errorHandling.fallbackNodeId` 的退役迁移文案把 fault 边写成
「an edge with condition 'fault'」。但 `condition` 是返回 boolean 的 CEL
谓词(ExpressionInputSchema),fault 路由在 `type`
(z.enum(['default','fault','conditional','back']))。

照原文案写 `{ source, target, condition: 'fault' }` 能 parse 通过 ——
`condition` 接受任意表达式字符串 —— 但得到的是一条普通边,不是 fault 边。
于是墓碑用一个静默失效的键换掉了另一个:作者删掉一个本就不存在的 fallback,
再画一条并非 fault 的「fault 边」。

仓库别处已写对(flows.mdx:869「type: 'fault' is what routes — a label is
not」),全树其余提法均为 `type: 'fault'`,此处是唯一一处走样。末句
「draw a fault edge from the failing node to the handler node instead」
本就正确,保持不变。

仅文案,无 schema / 校验 / 运行时行为变化。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011M7UwH25Unfi73UHim7ajY

Author: claude

.changeset/fault-edge-tombstone-type.md

@@ -0,0 +1,29 @@
+---
+"@objectstack/spec": patch
+---
+
+fix(spec): the `fallbackNodeId` tombstone names the key that actually routes faults (#6094)
+
+`flow.errorHandling.fallbackNodeId` was retired in 17.0.0 (#3896), and its
+migration message tells the author what to draw instead. It named the wrong key:
+
+- FROM: "the engine routes unrecoverable node errors via per-node fault edges
+  (an edge with **condition `'fault'`**)"
+- TO: "… (an edge with **`type: 'fault'`**)"
+
+`condition` on `FlowEdgeSchema` is a **CEL predicate** returning boolean
+(`flow.zod.ts` — `ExpressionInputSchema`), while the fault/default/conditional/back
+routing lives on `type` (`z.enum([...])`). An author following the old wording
+verbatim would write `{ source, target, condition: 'fault' }`, which **parses
+clean** — `condition` accepts any expression string — and produces an ordinary
+edge that is not a fault path. So the tombstone handed them a second silently
+inert key in exchange for the one it took away: they delete a fallback that never
+existed, then draw a fault edge that isn't one.
+
+The repo already states the correct rule elsewhere (`flows.mdx`: "`type: 'fault'`
+is what routes — a label is not"), and every other mention in the tree spells it
+`type: 'fault'`; this was the only site out of step. The closing sentence
+("draw a fault edge from the failing node to the handler node instead") was
+already correct and is unchanged.
+
+Message text only — no schema, validation, or runtime behaviour changes.

packages/spec/src/automation/flow.zod.ts

@@ -712,7 +712,7 @@ export const FlowSchema = lazySchema(() => z.object({
     fallbackNodeId: retiredKey(
       '`flow.errorHandling.fallbackNodeId` was removed in @objectstack/spec 17.0.0 (#3896 ' +
       'audit close-out) — the engine routes unrecoverable node errors via per-node fault ' +
-      "edges (an edge with condition 'fault'), and never read this key: a fallback " +
+      "edges (an edge with type: 'fault'), and never read this key: a fallback " +
       'configured here silently did not exist. Delete the key and draw a fault edge from ' +
       'the failing node to the handler node instead.',
     ),