feat(studio): stage 7 step 3c — sdk cutover for inline-style ops (#1522)

Introduces sdkCutoverPersist(): when STUDIO_SDK_CUTOVER_ENABLED is set,
inline-style PatchOps are routed through the SDK session's in-memory document
model instead of the server patch-element API. The SDK serialize() result is
written back through the same writeProjectFile + editHistory.recordEdit path,
so the on-disk output is identical to the legacy route.

- packages/studio/src/utils/sdkCutover.ts (new): sdkCutoverPersist() +
  shouldUseSdkCutover() guard; domEditSaveTimestampRef.current is stamped on
  each write to suppress the echo file-change reload.
- packages/studio/src/components/editor/manualEditingAvailability.ts: adds
  STUDIO_SDK_CUTOVER_ENABLED flag (default false); changes
  STUDIO_SDK_SHADOW_ENABLED default to false now that cutover is available.
- packages/studio/src/hooks/useSdkSession.ts: adds optional
  domEditSaveTimestampRef param; self-write suppress window (SELF_WRITE_SUPPRESS_MS)
  gates file-change reloads so SDK writes don't echo back as external edits.
- packages/studio/src/App.tsx: passes domEditSaveTimestampRef to useSdkSession
  so the suppress window can gate reloads triggered by SDK cutover writes.
- Test coverage: sdkCutover.test.ts (new, 141 lines) + useDomEditSession.test.ts
  (new, 50 lines) — guard function + happy-path assertions.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/studio/src/hooks/useDomEditSession.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vitest";
+import { shouldUseSdkCutover } from "../utils/sdkCutover";
+import type { PatchOperation } from "../utils/sourcePatcher";
+
+const styleOp = (property: string, value: string): PatchOperation => ({
+  type: "inline-style",
+  property,
+  value,
+});
+
+const attrOp = (property: string, value: string): PatchOperation => ({
+  type: "attribute",
+  property,
+  value,
+});
+
+describe("shouldUseSdkCutover", () => {
+  it("returns false when flag is disabled", () => {
+    expect(shouldUseSdkCutover(false, true, "hf-abc", [styleOp("color", "red")])).toBe(false);
+  });
+
+  it("returns false when no SDK session", () => {
+    expect(shouldUseSdkCutover(true, false, "hf-abc", [styleOp("color", "red")])).toBe(false);
+  });
+
+  it("returns false when selection has no hfId", () => {
+    expect(shouldUseSdkCutover(true, true, null, [styleOp("color", "red")])).toBe(false);
+    expect(shouldUseSdkCutover(true, true, undefined, [styleOp("color", "red")])).toBe(false);
+  });
+
+  it("returns false when ops array is empty", () => {
+    expect(shouldUseSdkCutover(true, true, "hf-abc", [])).toBe(false);
+  });
+
+  it("returns true when all conditions met with supported op types", () => {
+    expect(shouldUseSdkCutover(true, true, "hf-abc", [styleOp("color", "red")])).toBe(true);
+    expect(
+      shouldUseSdkCutover(true, true, "hf-abc", [styleOp("color", "red"), attrOp("data-x", "1")]),
+    ).toBe(true);
+  });
+});