[vitormattos]

Summary

Implements deterministic whole-feature sharding for #8974, without enabling parallel CI or altering Nextcloud installations.

• Selects sorted repository-relative .feature paths using zero-based SHARD_INDEX and positive SHARD_TOTAL; stable for the same revision and shard count.
• tests/integration/sharding/behat-shard.php supports --list, --verify (offline), and --run (one Behat process with only that shard's feature paths).
• Uses Behat's native multiple-path arguments and default configuration discovery; no custom Behat extension, tags or workflow modifications.
• Empty shards never run the full suite. --verify handles large totals without iterating millions of empty shards.
• Moves PHPUnit coverage into tests/php/Unit/Behat/ so existing composer test:unit jobs discover it; fixes PHP-CS-Fixer violations.

Verification actually performed

Check	Result
Standalone PHPUnit 10.5.66, PHP 8.2.35	34 tests / 91 assertions passed
Standalone PHPUnit 10.5.66, PHP 8.3.36	34 tests / 91 assertions passed
Project PHP CS Fixer 3.95.21, PHP 8.3	0 violations in all three affected PHP files
PHP lint, PHP 8.3	3 files passed
--verify, real LibreSign feature tree	46/46; 1 shard: 46; 2: 23/23; 4: 12/12/11/11; no overlaps or omissions
--run, genuine Behat 3.34.0, disposable six-feature fixture	3/3 scenarios passed in shard 0/2; 3/3 in shard 1/2; 1/1 in shard 3/4
Invalid input, empty shard, process arguments / exit propagation, large shard total	Covered by PHPUnit
git diff --check	Passed

The functional Behat smoke test used the exact Behat version pinned in tests/integration/composer.lock but a self-contained fixture, not a full Nextcloud deployment. The full integration suite and concurrent workers were not executed locally; their existing GitHub checks run normally on the PR.

Design rationale / upstream

• Behat 3.23 release added multiple feature-path support. LibreSign currently locks Behat 3.34.0; Behat CLI documentation supports passing feature paths. Thus no plugin or Behat internal API is required.
• Moodle's parallel Behat documentation separates feature assignment from isolated installations, databases and data directories. Oro's Behat documentation similarly treats feature-level application state isolation separately from selection.

Scope boundary

This PR does not execute shards concurrently, provision databases, isolate Nextcloud installations, change GitHub workflows, migrate to Behat 4, or add shard tags. This merge alone will not speed up CI. All mutable state/worker lifecycle and parallel execution remain #8975. There is no claim that selecting files makes shared Nextcloud installations safe to run concurrently.

Refs #8974, part of #8969.