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>

scripts/build_env.sh

@@ -27,4 +27,10 @@ if [ ! -d "$PY_VENV_DIR" ]; then
     python3 -m venv "$PY_VENV_DIR"
     source "$PY_VENV_DIR/bin/activate"
     pip install --extra-index-url https://pypi.python.org/pypi -r $REQ_PATH
+    # Validation suite plugin used to verify the report-exporter
+    # entry-points discovery end-to-end.
+    FAKE_EXPORTER_DIR="$(dirname "$REQ_PATH")/../test/validation/fake_exporter"
+    if [ -d "$FAKE_EXPORTER_DIR" ]; then
+        pip install -e "$FAKE_EXPORTER_DIR"
+    fi
 fi