fix(#41): validate caller-supplied DirectoryLayer::create prefix

[s2x]

Fixes #41

Summary

DirectoryLayer::create(...,$prefix) previously wrote any caller-supplied
prefix directly into the subdirs index without any conflict check, so a
manual prefix overlapping existing directory metadata or content keys
silently corrupted or overwrote data.

This change routes every caller-supplied prefix through
DirectoryLayer::validateRawPrefix(), which throws DirectoryException
with a printable rendering of the offending key when:

• the prefix is empty;
• the node-metadata range under the prefix is not empty;
• the content-key range under the prefix is not empty.

The check runs inside the transaction before any set() call, so a failed
create leaves no partial state and the same path can be retried via
auto-allocation.

Changes

• src/Directory/DirectoryLayer.php: Validation logic extracted into a
  private helper validateRawPrefix($rawPrefix,$contentSubspaceKey, $nodeProbe,$contentProbe) so it can be unit-tested in pure PHP
  without needing a live Transaction. The instance-level adapter
  assertValidCallerSuppliedPrefix($tr,$prefix) binds it to live
  FoundationDB probes. Also tightened the existing auto-allocation error
  message ("data" → "directory metadata") for consistency with the
  new explicit-prefix diagnostics.
• docs/directory-layer.md: Documents the explicit-prefix API and
  the three rejection conditions in a four-row table.
• CHANGELOG.md: New [Unreleased] entry under Fixed with the
  bug description, the validation contract, and a pointer to the two
  new test files.

Tests

• tests/Unit/DirectoryPrefixValidationTest.php (20 new tests):
  acceptance boundary (' visited, non-empty prefix accepted, 1-byte
  prefix accepted, binary prefix accepted, content-key composition
  observed at probes), rejection boundaries (empty prefix, metadata
  conflict, content conflict, ordering diagnostic when both conflict),
  error-message printable-rendering across printable, control, ~,
  0xFF, mixed.
• tests/Integration/DirectoryTest.php (4 new tests):
  end-to-end happy-path with auto-validation free prefix (open
  round-trip, usable for normal CRUD), two-distinct-prefixes
  acceptance, empty-prefix rejection (and no transaction state
  mutated afterwards), explicit-prefix overlapping content keys
  rejected.

All checks green locally:

composer lint   # PHPCS + Rector (dry-run) + PHPStan — `[OK] No errors`
composer test:unit  # OK (400 tests, 838 assertions)

CI will additionally exercise PHP 8.2/8.3/8.4 unit tests plus the
5-node FDB cluster e2e-tests job (which the integration suite
runs).