feat(fleet): mosaic fleet regen — regenerate roster-derived projections (PR3 of #791) (#813)

packages/mosaic/src/fleet/generated-env-boundary.ts

@@ -204,6 +204,53 @@ export async function prepareAgentEnvironmentProjection(
   };
 }
 
+/** A generated-only projection validated without touching any legacy/local/quarantine file. */
+export interface PreparedGeneratedAgentEnvironmentProjection {
+  readonly mosaicHome: string;
+  readonly agentEnvDir: string;
+  readonly generatedPath: string;
+  readonly generated: string;
+}
+
+/**
+ * Validates ONLY the roster-derived generated projection for a recovery rebuild.
+ * Unlike {@link prepareAgentEnvironmentProjection}, this never reads, classifies,
+ * relocates, or quarantines the legacy `.env` / `.env.local` operator surface — it
+ * exists so `mosaic fleet regen` has no code path that can mutate anything except
+ * `<name>.env.generated`. The existing generated file, if present, must already be
+ * a private regular file.
+ */
+export async function prepareGeneratedAgentEnvironmentProjection(
+  options: AgentEnvironmentProjectionOptions,
+): Promise<PreparedGeneratedAgentEnvironmentProjection> {
+  if (!AGENT_NAME.test(options.agentName)) {
+    throw new AgentEnvBoundaryError('unsafe-agent-name', 'MOSAIC_AGENT_NAME', options.agentName);
+  }
+  await validatePrivateProjectionDirectory(options.mosaicHome, options.agentEnvDir);
+  const generatedPath = join(options.agentEnvDir, `${options.agentName}.env.generated`);
+  const generated = renderGeneratedAgentEnvironment(options.generated);
+  await assertPrivateRegularFileIfPresent(generatedPath);
+  return {
+    mosaicHome: options.mosaicHome,
+    agentEnvDir: options.agentEnvDir,
+    generatedPath,
+    generated,
+  };
+}
+
+/**
+ * Applies a generated-only projection: writes ONLY `<name>.env.generated` atomically
+ * and touches nothing else. It never writes `.env.local`/`.env.quarantine` and never
+ * unlinks the legacy `.env` — the projection-only recovery guarantee is structural.
+ */
+export async function applyPreparedGeneratedAgentEnvironmentProjection(
+  prepared: PreparedGeneratedAgentEnvironmentProjection,
+): Promise<string> {
+  await ensurePrivateProjectionDirectory(prepared.mosaicHome, prepared.agentEnvDir);
+  await writePrivateAtomically(prepared.generatedPath, prepared.generated);
+  return prepared.generatedPath;
+}
+
 /**
  * Validates only the exact generated projection eligible for a roster delete.
  * Local overrides, legacy input, and quarantine records are operator-retained and