feat(#93): IMP-55 u1~u12 frontend manual section swap detection (manual_section_assignment bool axis + drag-only marker gate + dual-axis persistence + backend manual-true gate)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Front/client/tests/user_overrides_endpoint.test.ts

@@ -310,16 +310,44 @@ describe("KNOWN_USER_OVERRIDES_AXES (IMP-52 u4)", () => {
     // The on-disk schema is shared with backend pipeline fallback (u2).
     // Any drift here means a PUT could write an axis that the Python
     // load() ignores, or vice-versa, silently losing user overrides.
+    // Mirror is Python-minus-`slide_css` (known IMP-45 #74 gap — the
+    // frontend never writes slide_css). IMP-55 #93 u1 adds the bool
+    // `manual_section_assignment` axis as a first-class allowlist entry.
     expect(KNOWN_USER_OVERRIDES_AXES).toEqual([
       "layout",
       "zone_geometries",
       "zone_sections",
       "frames",
       "image_overrides",
+      "manual_section_assignment",
     ]);
   });
 });
 
+describe("mergeUserOverrides (IMP-55 #93 u1) — manual_section_assignment bool axis", () => {
+  it("merges bool true / false literally and clears on null", () => {
+    // The PUT handler must treat the bool axis like any other allowlisted
+    // axis: replace on write, preserve when absent, delete on null. Tests
+    // both true→false flip and explicit null-clear so the backend (u9)
+    // sees the exact frontend intent.
+    let merged = mergeUserOverrides({}, { manual_section_assignment: true });
+    expect(merged.manual_section_assignment).toBe(true);
+
+    merged = mergeUserOverrides(merged, { manual_section_assignment: false });
+    expect(merged.manual_section_assignment).toBe(false);
+
+    merged = mergeUserOverrides(merged, { manual_section_assignment: null });
+    expect("manual_section_assignment" in merged).toBe(false);
+  });
+
+  it("preserves bool axis when partial touches only a sibling axis", () => {
+    const existing = { manual_section_assignment: true, layout: "old" };
+    const merged = mergeUserOverrides(existing, { layout: "new" });
+    expect(merged.manual_section_assignment).toBe(true);
+    expect(merged.layout).toBe("new");
+  });
+});
+
 describe("mergeUserOverrides (IMP-52 u4)", () => {
   it("only mutates KNOWN_AXES present in partial", () => {
     const existing = {