Fix is_readable() to test readability, not writability

[remyperona]

Summary

VirtualFilesystemDirect::is_readable() delegated to is_writeable(), so it returned an entry's writability instead of its readability. A read-only entry (0444) is readable but not writable, and was wrongly reported as "not readable".

This is not cosmetic: dirlist() gates on is_readable(), so listing a read-only directory returned false — a false negative for exactly the permission scenario this filesystem mock exists to reproduce.

Change

public function is_readable( $file ) {
-	return is_writeable( $this->getUrl( $file ) );
+	return is_readable( $this->getUrl( $file ) );
}

Regression coverage

• isReadable.php: 0444 asserts true
• isWritable.php: 0444 asserts false (guards against the inverse regression)
• dirlist.php: a read-only directory still returns its listing, not false

The pre-existing unit tests only covered 0777 (→ true) and 000 (→ false), both of which passed under the buggy delegation, which is why this went unnoticed.

Validation

Ran the full suite in Docker via wp-env (WordPress test site, PHP 8.3.33, PHPUnit 9.6.35):

• Unit: 93 tests, 284 assertions — OK
• Integration: 29 tests, 70 assertions — OK
• Integration (AdminOnly): 3 tests, 4 assertions — OK

Both new read-only cases were confirmed to execute (not silently skipped).

Fixes #34

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