docs(phpdoc): document public methods across src/

Add professional PHPDoc (summary + @param/@return) to the remaining
public methods that had none or only partial coverage: GraphController's
actions, ContributionStore's accessors, SvgRenderer's render() and
private helpers (with concrete array-shape annotations), ProbeTrait,
ProviderHealthChecker, ProviderStatus, and the Contribution entity.
Also adds the matching @return shapes on SvgRendererTest's data
providers.

tests/Unit/Service/Renderer/SvgRendererTest.php

@@ -54,6 +54,7 @@ final class SvgRendererTest extends TestCase
         $this->assertStringContainsString($expectedColor, $svg);
     }
 
+    /** @return iterable<string, array{string, string}> */
     public static function theme_background_provider(): iterable
     {
         yield 'dark theme'    => ['dark',    '#0d1117'];
@@ -97,6 +98,7 @@ final class SvgRendererTest extends TestCase
         $this->assertStringContainsString('>' . $label . '<', $svg);
     }
 
+    /** @return iterable<string, array{string}> */
     public static function day_of_week_label_provider(): iterable
     {
         yield 'Monday'    => ['Mon'];