feat(FN-7360): remove legacy workflow step engine

The step execution engine was already gone (runWorkflowSteps deleted,
workflow_steps table dropped in migration 132). This removes what remained:
the linear step compiler (compileWorkflowToSteps/validateLinearity/
WorkflowCompileError), which survived only as a validator + step-preview
generator.

parseWorkflowIr/validateV2 (which accepts branching graphs) is now the sole
workflow validity gate at save/select/refine and in the graph task runner.
Custom branching workflows are now selectable and run on the graph
interpreter instead of being rejected as non-linear.

- core: delete workflow-compiler.ts; rework store.validateWorkflowCompilable
  onto parseWorkflowIr; move MERGE_REGION_NODE_KINDS into
  workflow-lifecycle-validation; retag workflow-steps-to-ir as legacy lowering
- engine: drop the compiler double-validation in workflow-graph-task-runner
- dashboard: remove POST /api/workflows/:id/compile + client wrapper; drop the
  interpreterOnly response field and editor banner; no post-save compile check
- i18n: remove the orphaned workflowNodes.interpreterOnly key across locales
- tests: reframe two workflow-selection tests whose premise inverted; fix a
  pre-existing red in builtin-lead-generation (completion-summary node)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/dashboard/app/api/legacy.ts

@@ -16,7 +16,6 @@ import type {
   ActivityLogEntry,
   ActivityEventType,
   WorkflowStep,
-  WorkflowStepInput,
   WorkflowStepResult,
   PluginInstallation,
   PluginSetupCheckResult,
@@ -5487,13 +5486,6 @@ export function updateWorkflowPromptOverrides(
   );
 }
 
-/** Preview the compiled steps for a workflow. Rejects (422) for non-linear graphs. */
-export function compileWorkflow(id: string, projectId?: string): Promise<{ steps: WorkflowStepInput[] }> {
-  return api<{ steps: WorkflowStepInput[] }>(withProjectId(`/workflows/${encodeURIComponent(id)}/compile`, projectId), {
-    method: "POST",
-  });
-}
-
 /** A workflow export envelope (U5/R9/KTD-5). `schemaVersion` is the SERVER's
  *  schema version at export time — the import route version-gates against it
  *  (the app build aliases @fusion/core to types-only, so the value can only come
@@ -5561,13 +5553,12 @@ export function importWorkflow(
 // MigrateLegacyStepsResult along with the legacy workflow_steps table and its route.
 
 /** Result of POST /api/workflows/design (U10/R11). The server validates the
- *  AI-produced IR (parseWorkflowIr), triages compilability (`interpreterOnly`),
- *  and strips trust-escalating flags (`strippedApprovalFlags`). Persists nothing
- *  — the client decides what to do with the returned graph. */
+ *  AI-produced IR (parseWorkflowIr) and strips trust-escalating flags
+ *  (`strippedApprovalFlags`). Persists nothing — the client decides what to do
+ *  with the returned graph. */
 export interface DesignWorkflowResult {
   ir: import("@fusion/core").WorkflowIr;
   layout: import("@fusion/core").WorkflowDefinition["layout"];
-  interpreterOnly: boolean;
   strippedApprovalFlags: boolean;
 }