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/items/report_plugin/file_check.py

@@ -0,0 +1,8 @@
+import os
+
+
+def file_contains(path, text):
+    if not os.path.isfile(path):
+        return False
+    with open(path, 'r') as f:
+        return text in f.read()

test/validation/items/report_plugin/param.yaml

@@ -0,0 +1 @@
+no_param: Null

test/validation/items/report_plugin/test.tum

@@ -0,0 +1,23 @@
+- report:
+    name: Unknown exporter is skipped (must pass)
+    key: $(test)_PASS
+    export:
+        - definitely_not_a_format:
+            path: $(validation_report_path)$(psep)$(test)_unknown.txt
+
+- report:
+    name: Plugin exporter from entry-points (fake_format CSV)
+    key: $(test)_PASS
+    export:
+        - fake_format:
+            path: $(validation_report_path)$(psep)$(test)_fake.csv
+
+- py_func:
+    name: Check fake_format CSV content
+    file: $(test_path)$(psep)file_check.py
+    func_name: file_contains
+    key: $(test)_PASS
+    param:
+        - $(validation_report_path)$(psep)$(test)_fake.csv
+        - "Test preparation,Group"
+    expected_result: True