test: add PHPUnit 11.5 unit test suite

- phpunit.xml.dist with unit suite and source coverage config
- .phpunit.cache excluded in .gitignore
- Initial SvgRendererTest as example test
- .claude/settings.json with PostToolUse hook to auto-run phpunit on edit

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

phpunit.xml.dist

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/11.5/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache"
+>
+    <testsuites>
+        <testsuite name="unit">
+            <directory>tests/Unit</directory>
+        </testsuite>
+    </testsuites>
+
+    <source>
+        <include>
+            <directory suffix=".php">src</directory>
+        </include>
+    </source>
+</phpunit>