M1: PHPUnit test suite for crypto, repository and resumable runs

Dev tooling via composer (phpunit, psalm, nextcloud/ocp); vendor/ is
mounted from outside the repository by a helper script. 28 unit tests
ported from the phase-0 spikes (round trips, corruption, wrong keys,
dedup, incremental runs, partial restore, tampering, object binding,
time-boxed steps, file changed between steps).

tests/phpunit.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="../vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="bootstrap.php" colors="true" failOnWarning="true" failOnRisky="true">
+	<testsuites>
+		<testsuite name="unit">
+			<directory>Unit</directory>
+		</testsuite>
+	</testsuites>
+</phpunit>