Studio diffusion (Phase 6) review round 2: img2img source dims + upscale repeats

Codex review on the native engine arg builder:

- build_sd_cpp_command emitted --width/--height unconditionally, so an
  img2img/inpaint/edit run that left dims unset forced a 1024x1024 resize/crop of
  the input. width/height are now Optional (None = unset): an image-conditioned
  run (init_img or ref_images) with unset dims omits the flags so sd.cpp derives
  the size from the input image (set_width_and_height_if_unset); a plain txt2img
  run with unset dims keeps the prior 1024x1024 default; explicit dims are always
  honored. width/height are read only by the builder, so the type change is local.

- build_sd_cpp_upscale_command used a truthiness guard (params.repeats and ...)
  that silently swallowed repeats=0 into sd-cli's default of one pass, turning an
  explicit no-op into a real upscale. It now rejects repeats < 1 with ValueError
  and emits the flag for any explicit value != 1.

Tests: img2img unset dims omit width/height (init_img and ref_images), explicit
dims emitted, txt2img keeps 1024; upscale rejects repeats=0 and omits the flag at
the default. (Two pre-existing binary-discovery tests fail only because a real
sd-cli is installed in this dev environment; unrelated to this change.)

studio/backend/tests/test_sd_cpp_args.py

@@ -221,6 +221,47 @@ def test_build_edit_repeats_ref_image():
     assert [cmd[i + 1] for i in idxs] == ["/r/a.png", "/r/b.png"]
 
 
+def test_img2img_unset_dims_lets_sdcpp_derive_from_source():
+    # img2img/inpaint/edit with dims left unset must NOT force --width/--height,
+    # so sd.cpp derives the size from the input image instead of resizing it to 1024.
+    files = SdCppModelFiles(diffusion_model = "/m/z.gguf")
+    cmd = build_sd_cpp_command(
+        "/bin/sd-cli",
+        files,
+        SdCppGenParams(prompt = "x", init_img = "/in/src.png"),
+        output_path = "/o.png",
+    )
+    assert "--width" not in cmd and "--height" not in cmd
+    # an edit (ref-image) run derives its size too
+    cmd2 = build_sd_cpp_command(
+        "/bin/sd-cli",
+        files,
+        SdCppGenParams(prompt = "x", ref_images = ("/r/a.png",)),
+        output_path = "/o.png",
+    )
+    assert "--width" not in cmd2 and "--height" not in cmd2
+
+
+def test_img2img_explicit_dims_are_emitted():
+    files = SdCppModelFiles(diffusion_model = "/m/z.gguf")
+    cmd = build_sd_cpp_command(
+        "/bin/sd-cli",
+        files,
+        SdCppGenParams(prompt = "x", init_img = "/in/src.png", width = 768, height = 512),
+        output_path = "/o.png",
+    )
+    assert _pair(cmd, "--width") == "768" and _pair(cmd, "--height") == "512"
+
+
+def test_txt2img_unset_dims_keep_1024_default():
+    # A plain txt2img run with no dims keeps the prior 1024x1024 default.
+    files = SdCppModelFiles(diffusion_model = "/m/z.gguf")
+    cmd = build_sd_cpp_command(
+        "/bin/sd-cli", files, SdCppGenParams(prompt = "x"), output_path = "/o.png"
+    )
+    assert _pair(cmd, "--width") == "1024" and _pair(cmd, "--height") == "1024"
+
+
 def test_build_lora_dir_and_apply_mode():
     files = SdCppModelFiles(diffusion_model = "/m/z.gguf")
     params = SdCppGenParams(
@@ -261,6 +302,25 @@ def test_build_upscale_command():
     assert "--prompt" not in cmd and "--llm" not in cmd
 
 
+def test_build_upscale_rejects_non_positive_repeats():
+    # repeats=0 must not be silently swallowed into sd-cli's default of one pass.
+    with pytest.raises(ValueError, match = "repeats"):
+        build_sd_cpp_upscale_command(
+            "/bin/sd-cli",
+            SdCppUpscaleParams(input_image = "/i.png", upscale_model = "/m/e.pth", repeats = 0),
+            output_path = "/o.png",
+        )
+
+
+def test_build_upscale_default_repeats_omits_flag():
+    cmd = build_sd_cpp_upscale_command(
+        "/bin/sd-cli",
+        SdCppUpscaleParams(input_image = "/i.png", upscale_model = "/m/e.pth"),  # repeats=1
+        output_path = "/o.png",
+    )
+    assert "--upscale-repeats" not in cmd
+
+
 def test_build_upscale_requires_input_and_model():
     with pytest.raises(ValueError):
         build_sd_cpp_upscale_command(