fix: prevent crash converting UUID timestamps near the Unix epoch

PhpTimeConverter::convertTime() threw an uncaught
InvalidArgumentException for v1/v6 UUID timestamps within about
100 microseconds of the Unix epoch (on the pre-1970 side), because
PHP renders such small floats in scientific notation, which
splitTime() misparsed as a malformed microseconds fragment.

splitTime() now recognizes scientific notation and defers to the
existing GenericTimeConverter fallback, the same mechanism already
used for other precision-loss cases.

tests/Converter/Time/PhpTimeConverterTest.php

@@ -133,6 +133,14 @@ class PhpTimeConverterTest extends TestCase
                 'unixTimestamp' => '0',
                 'microseconds' => '0',
             ],
+
+            // One microsecond before the epoch: PHP renders "-1.0E-6", which must not
+            // be misparsed as microseconds.
+            [
+                'uuidTimestamp' => new Hexadecimal('1b21dd213813ff6'),
+                'unixTimestamp' => '0',
+                'microseconds' => '1',
+            ],
         ];
     }