Adds 134 pytest tests organized into nine files under ``tests/``,
covering the entire WireViz API surface from parse() through CLI to
BOM and color logic. Every upstream PR we ported and every gemini
review fix gets a pinned regression test in tests/test_regressions.py
so future refactors can't silently undo the work.

## Test files (134 tests total)

* tests/test_smoke.py (10)        — every output format renders
* tests/test_parse.py (20)        — wireviz.parse() API surface
* tests/test_cli.py (19)          — CLI flags via Click's CliRunner
* tests/test_harness.py (11)      — Harness public methods + render
* tests/test_dataclasses.py (19)  — DataClasses coercion + validation
* tests/test_colors.py (12)       — color schemes + hex + thickness
* tests/test_bom.py (8)           — BOM aggregation
* tests/test_regressions.py (29)  — one per ported PR + review fix
* tests/test_round_trip.py (6)    — PNG embed + stdin/stdout

## Real bugs surfaced and fixed

1. **wv_html.py:72** — ``re.sub(..., 1)`` triggered Python 3.13+'s
   "positional 'count' is deprecated" DeprecationWarning. Switched to
   ``count=1``. Would have started failing CI on Python 3.13 once
   the warning becomes an error.

2. **wv_cli.py "File does not exist"** — was a bare ``raise Exception(...)``
   that produced a Python traceback for users who pointed the CLI at a
   missing input or prepend file. Promoted both sites to
   ``click.UsageError`` so users see the canonical
   "Try 'wireviz -h' for help" footer instead.

3. **wireviz.py:_get_yaml_data_and_path** — when called with a dict
   input, parse() would mutate the caller's dict in place during
   connection-set expansion (``[[{X1: [1,2]}, ...]]`` got rewritten
   to ``[[{X1: 1}, {X1: 2}, ...]``). Added ``copy.deepcopy(inp)`` so
   programmatic callers (especially the planned wireviz-gui that
   will keep an in-memory model) get clean read-only semantics.

A regression test exists in tests/test_round_trip.py
(``test_dict_input_not_mutated``) so this can't quietly come back.

## Infrastructure

* pyproject.toml gains ``[tool.pytest.ini_options]`` with
  ``testpaths = ["tests"]`` and ``filterwarnings = ["error",
  "ignore::SyntaxWarning"]`` so pytest catches future deprecation drift.
* tests/conftest.py centralizes fixtures pointing at small YAMLs in
  tests/fixtures/ — deliberately distinct from the gallery YAMLs in
  examples/ so tests don't break when the gallery shifts.
* New .github/workflows/tests.yml runs pytest across the same
  Python 3.7-3.12 matrix as the existing Create Examples workflow.

## Also folded in: cherry-pick of e4f485e (PR #10 review fixes)

The PR #10 review-fix commit (data URIs without leading space, click
.UsageError on unknown -f code, image_paths default for stdin,
better tweak conflict error message, source_path auto-fill in parse())
was on port-321-stdin-stdout but never reached master because PR #10
merged before that commit was pushed. Cherry-picked here so the
test suite can validate the fixed behavior.

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

The ``_get_yaml_data_and_path`` return-type annotation
``Tuple[Dict, Optional[Path], Optional[str]]`` references ``Optional``
which wasn't in the typing import line.

This worked locally on Python 3.14 because of PEP 649 deferred
annotations, but every CI runner (3.7-3.12) evaluates annotations at
module-import time and crashes with::

  NameError: name 'Optional' is not defined

Both the Tests workflow (pytest) and the Create Examples workflow
(build_examples.py) failed for this reason — both import the wireviz
package at startup.

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

Reproducing the remaining CI failures locally with ``pip install
'click<8.2'`` showed Click 8.1.x's CliRunner with ``mix_stderr=False``
captures ``click.echo(..., err=True)`` only — direct
``sys.stderr.write()`` calls bypass its capture and never reach
``result.stderr``. Click 8.2+ closed that gap and now captures both,
which is why the local dev venv (Click 8.2.1) passed.

Tests asserting on the WireViz banner ("WireViz 0.4.1") and per-file
status lines ("Input file: ...", "Output: <stdout>.svg",
"(extracted YAML)") all read empty stderr on the three CI matrix
Pythons (3.7/3.8/3.9) where pip resolved Click 8.1.x.

Switched all eight ``sys.stderr.write`` sites in wv_cli.py to
``click.echo(message, err=True)``. That's the idiomatic Click pattern
and works uniformly on every Click 8.x release. Library-level prints
in DataClasses.py / Harness.py / wv_helper.py / wv_colors.py
intentionally *keep* using ``sys.stderr.write`` — those modules
mustn't import Click since they're consumable by non-CLI library
callers (and will be by the planned wireviz-gui).

Verified: 134/134 passing locally on Click 8.1.8 (matches the
failing CI matrix entries) and on Click 8.3+.

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