feat(#90): IMP-56 u1-u19 catch-up before final close (post-u20 push fix)

u1: text_overrides axis in user_overrides_io
u2: structure_overrides axis in user_overrides_io
u3: vite allowlist for new endpoints
u4: text_override_resolver
u5: Step 12 text_overrides apply in phase_z2_pipeline
u6: structure_override_resolver
u7: text_path_stamper
u8: SlideCanvas text-edit capture
u9: SlideCanvas structure-edit overlay
u10: userOverridesApi service extension
u11: designAgent types extension
u12: slidePlanUtils restore
u13: user_overrides endpoint tests
u14: user_overrides restore tests
u15: pipeline fallback tests
u16: edit-mode state + gating tests
u17: slide_base print mode CSS
u18: /api/connect endpoint (vite)
u19: /api/export endpoint (vite)

Recovery scope: 29 files (12 modified + 17 new). u20 already pushed in
9439575; this commit lands u1-u19 that were authored but not committed
before #90 was externally closed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_user_overrides_pipeline_fallback.py

@@ -55,6 +55,13 @@ def _exec_main_block(
         # the section-assignment axis; the new kwargs are captured here so any
         # follow-up test can pin them without re-touching this harness.
         override_slide_css=None,
+        # IMP-56 (#90) u16 — absorb the two new file-only axes added to
+        # ``run_phase_z2_mvp1`` so the existing harness keeps working when
+        # the CLI dispatch passes the new kwargs through. Both default to
+        # ``None`` so the no-file / corrupt-file / invalid-stem tests can
+        # continue asserting "all overrides None".
+        override_text_overrides=None,
+        override_structure_overrides=None,
         reuse_from=None,
     ):
         captured["mdx_path"] = mdx_path
@@ -65,6 +72,8 @@ def _exec_main_block(
         captured["override_section_assignments"] = override_section_assignments
         captured["override_image_overrides"] = override_image_overrides
         captured["override_slide_css"] = override_slide_css
+        captured["override_text_overrides"] = override_text_overrides
+        captured["override_structure_overrides"] = override_structure_overrides
         captured["reuse_from"] = reuse_from
 
     monkeypatch.setattr(_pz2, "run_phase_z2_mvp1", _fake_run)
@@ -600,3 +609,69 @@ def test_cli_section_assignment_works_without_persisted_file(
     )
 
     assert captured["override_section_assignments"] == {"top": ["cli-only"]}
+
+
+# -- 6. IMP-56 (#90) u16 — text_overrides + structure_overrides file fallback
+
+
+def test_file_text_overrides_flow_through_when_no_cli(tmp_path, monkeypatch):
+    """text_overrides axis is file-only — JSON payload reaches run kwarg."""
+    _redirect_overrides_root(tmp_path, monkeypatch)
+    (tmp_path / "03.json").write_text(
+        json.dumps({
+            "text_overrides": {
+                "top": {"title.0": "edited title", "body.1": "edited line"},
+                "bottom_l": {"caption.0": "edited caption"},
+            },
+        }),
+        encoding="utf-8",
+    )
+
+    captured: dict[str, Any] = {}
+    _exec_main_block(
+        captured, ["src.phase_z2_pipeline", "03.mdx"], monkeypatch,
+    )
+
+    assert captured["override_text_overrides"] == {
+        "top": {"title.0": "edited title", "body.1": "edited line"},
+        "bottom_l": {"caption.0": "edited caption"},
+    }
+    # No structure payload on file → kwarg collapses to None via ``or None``.
+    assert captured["override_structure_overrides"] is None
+
+
+def test_file_structure_overrides_flow_through_when_no_cli(
+    tmp_path, monkeypatch
+):
+    """structure_overrides axis is file-only; inner keys locked to
+    slot_order + hidden_slots (any other inner key is dropped by the
+    CLI gate). Non-string list elements are dropped too."""
+    _redirect_overrides_root(tmp_path, monkeypatch)
+    (tmp_path / "03.json").write_text(
+        json.dumps({
+            "structure_overrides": {
+                "top": {
+                    "slot_order": ["c", "a", "b"],
+                    "hidden_slots": ["d"],
+                    # foreign key — must be dropped by the CLI gate
+                    "frame_id": "swap_attempt",
+                    # non-string elements — must be dropped per-entry
+                    "slot_order_with_junk": ["x", 1, None, "y"],
+                },
+                "bottom": {"hidden_slots": ["e"]},
+            },
+        }),
+        encoding="utf-8",
+    )
+
+    captured: dict[str, Any] = {}
+    _exec_main_block(
+        captured, ["src.phase_z2_pipeline", "03.mdx"], monkeypatch,
+    )
+
+    assert captured["override_structure_overrides"] == {
+        "top": {"slot_order": ["c", "a", "b"], "hidden_slots": ["d"]},
+        "bottom": {"hidden_slots": ["e"]},
+    }
+    # No text payload on file → kwarg collapses to None.
+    assert captured["override_text_overrides"] is None