Add plugin registry for report exporters

Replace the hardcoded if/elif in Export.exec() with a dict registry.
Built-in formats (text, json, junit, html) are registered as lazy
loaders; missing optional deps (junit_xml, lxml) print a clear message
with a pip install hint instead of raising. Entry-points
(group "testium.exporters") are discovered at import time — installed
plugins are auto-detected with no extra config.

An unknown or unavailable format prints an info line and skips the
export; the test run is not interrupted.

Validation:
- New testium-fake-exporter package under test/validation/fake_exporter/
  installed automatically by scripts/build_env.sh on venv creation.
  It registers fake_format via entry-points and exports the tests
  table to CSV — a real, useful exporter that exercises the plugin
  contract end-to-end (entry-point discovery, dispatch, SQLite query).
- New dedicated items/report_plugin/ test exercises both the
  unknown-format skip path and the fake_format plugin path, with a
  py_func check (file_check.py) on the produced CSV. Runs once per
  validation suite.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/validation/fake_exporter/pyproject.toml

@@ -0,0 +1,14 @@
+[build-system]
+requires = ["setuptools>=61"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "testium-fake-exporter"
+version = "0.1.0"
+description = "Fake report exporter used by testium validation suite"
+
+[project.entry-points."testium.exporters"]
+fake_format = "fake_exporter:FakeExporter"
+
+[tool.setuptools.packages.find]
+where = ["."]