Address PR #10 review feedback: data URIs, errors, source_path auto-fill

Five gemini-code-assist comments on
#10:

* Data URI leading-space stripped in three places (svgembed.py
  data_URI_base64 and embed_svg_images, plus Harness._render's PNG
  b64 inline). The prior ``data:image/png;base64, <b64>`` form had a
  stray space after the comma which violates RFC 2397; most browsers
  tolerate it but stricter parsers (some PDF/EPUB tooling, lint tools)
  reject it. All three call sites now emit the canonical
  ``data:image/png;base64,<b64>``.

* wv_cli.py: ``raise Exception("Unknown output format: ...")`` →
  ``raise click.UsageError(...)``. Matches the multi-format-stdout
  check elsewhere in the file and gives users the clean
  ``Try 'wireviz -h' for help.\nError: ...`` UX instead of a Python
  traceback.

* wv_cli.py: stdin mode now defaults ``image_paths`` to
  ``{Path.cwd()}`` instead of an empty set, so relative
  ``image: src:`` references in piped YAML resolve against the
  invocation directory the same way file-based input resolves
  against the YAML's parent.

* Harness._extend_tweak: tweak override conflict error now reports
  both the existing and new values
  (``"X1.tweak.override.X1.color: new value 'blue' conflicts with
  existing 'red'"``) instead of the prior vague ``"conflicts with
  another"``.

* wireviz.parse(): ``source_path`` now auto-fills from ``yaml_file``
  when the input was a Path. The docstring already promised this
  behavior; the code didn't actually do it. Confirmed via
  ``parse(Path('examples/demo01.yml'), output_formats=('svg',), ...)``
  succeeding without explicit source_path.

Verified against build_examples.py: deterministic outputs (.gv,
.bom.tsv) byte-identical to baseline. Tweak conflict YAML correctly
raises the new error. Bad CLI format flag now renders Click's
UsageError formatting. Data URIs in re-rendered ex08.svg / .html now
emit ``data:image/png;base64,iVB...`` (no space).

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

Author: SomethingNew71

src/wireviz/Harness.py

@@ -172,7 +172,8 @@ def _extend_tweak(self, node: Union[Connector, Cable]) -> None:
                 k, v = rph(k), rph(v)
                 if k in s_dict and v != s_dict[k]:
                     raise ValueError(
-                        f"{node.name}.tweak.override.{ident}.{k} conflicts with another"
+                        f"{node.name}.tweak.override.{ident}.{k}: new value "
+                        f"{v!r} conflicts with existing {s_dict[k]!r}"
                     )
                 s_dict[k] = v
             # Keep the empty dict rather than collapsing to None — the
@@ -935,7 +936,7 @@ def _render(
                 # rendered in this same call; otherwise let the template
                 # fall back to reading {output_dir}/{output_name}.png.
                 png_b64 = (
-                    f"data:image/png;base64, {base64.b64encode(png_bytes).decode('utf-8')}"
+                    f"data:image/png;base64,{base64.b64encode(png_bytes).decode('utf-8')}"
                     if png_bytes is not None
                     else None
                 )

src/wireviz/svgembed.py

@@ -13,7 +13,7 @@ def data_URI_base64(file: Union[str, Path], media: str = "image") -> str:
     """Return Base64-encoded data URI of input file."""
     file = Path(file)
     b64 = base64.b64encode(file.read_bytes()).decode("utf-8")
-    uri = f"data:{media}/{get_mime_subtype(file)};base64, {b64}"
+    uri = f"data:{media}/{get_mime_subtype(file)};base64,{b64}"
     # print(f"data_URI_base64('{file}', '{media}') -> {len(uri)}-character URI")
     if len(uri) > 65535:
         print(
@@ -36,7 +36,7 @@ def replace(match: re.Match) -> str:
             images_b64[imgurl] = base64.b64encode(image).decode("utf-8")
         return image_tag(
             match["PRE"] or "",
-            f"data:image/{get_mime_subtype(imgurl)};base64, {images_b64[imgurl]}",
+            f"data:image/{get_mime_subtype(imgurl)};base64,{images_b64[imgurl]}",
             match["POST"] or "",
         )
 

src/wireviz/wireviz.py

@@ -112,6 +112,10 @@ def parse(
         raise TypeError(
             f"Expected a dict as top-level YAML input, but got: {type(yaml_data)}"
         )
+    # When inp was a Path, derive source_path automatically so callers
+    # don't have to pass it twice. Matches the docstring contract.
+    if source_path is None and yaml_file is not None:
+        source_path = yaml_file
     write_to_stdout = (
         output_formats and (str(output_dir) == "-" or str(output_name) == "-")
     )

src/wireviz/wv_cli.py

@@ -116,7 +116,7 @@ def wireviz(
             if fmt not in output_formats:
                 output_formats.append(fmt)
         else:
-            raise Exception(f"Unknown output format: {code}")
+            raise click.UsageError(f"Unknown output format: {code}")
     output_formats = tuple(output_formats)
     output_formats_str = (
         f'[{"|".join(output_formats)}]'
@@ -150,7 +150,11 @@ def wireviz(
     for file in filepaths:
         if str(file) == "-":
             yaml_input = prepend_input + sys.stdin.read()
-            image_paths = set()
+            # No source-file directory available, so any relative
+            # `image: src:` paths in the stdin YAML are resolved against
+            # the current working directory (matching how a typical
+            # piped invocation would be run from a project root).
+            image_paths = {Path.cwd()}
             sys.stderr.write("Input:        <stdin>\n")
             _output_dir = output_dir if output_dir else "-"
             _output_name = output_name if output_name else "stdin"