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.

src/Entity/Contribution.php

@@ -4,8 +4,14 @@ declare(strict_types=1);
 
 namespace GitContributionGraph\Entity;
 
+/** A single provider's contribution count for one day, as stored in {@see \GitContributionGraph\Service\ContributionStore}. */
 final class Contribution
 {
+    /**
+     * @param string $provider provider identifier, e.g. "github"
+     * @param int    $date     day of the contribution, as a unix timestamp
+     * @param int    $count    contribution count for that day
+     */
     public function __construct(
         public readonly string $provider,
         public readonly int $date,