Support nul log path on Windows

[clue]

This changeset adds support for passing nul as the log path on Windows and updates the test suite and documentation to use this instead of an absolute path like C:\…\nul. The latest PHP releases reject reserved Windows device names as part of a path as a fix for GHSA-9f67-6fw4-hpfp, so our Windows tests started failing with Unable to open log file on these versions. A plain nul works across all PHP versions, so this is now also what the documentation recommends to disable the access log on Windows.

$container = new FrameworkX\Container([
    'accesslog' => DIRECTORY_SEPARATOR !== '\\' ? '/dev/null' : 'nul',
    FrameworkX\AccessLogHandler::class => fn(string $accesslog) => new FrameworkX\AccessLogHandler($accesslog),
]);

Absolute nul paths continue to work on older PHP versions and remain covered by the test suite, so this should be safe to apply.

Builds on top of #247, #296, #31 and others

[Copilot]

Copilot review overview

🟢 Approval recommended

The only outstanding item is a minor documentation clarification; no blocking issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Adds support for using plain nul as the Windows log path, compatible with recent PHP versions.

Changes:

• Accepts case-insensitive nul on Windows.
• Updates tests and integration fixtures.
• Updates application documentation.

File	Summary
tests/​Io/​LogStreamHandlerTest.php	Adds Windows null-device path coverage.
tests/​integration/​public/​index.php	Uses plain nul for integration logging.
tests/​AppTest.php	Updates test logger configuration.
tests/​AppMiddlewareTest.php	Updates middleware test logger configuration.
tests/​AccessLogHandlerTest.php	Uses plain nul for disabled logging.
src/​Io/​LogStreamHandler.php	Recognizes Windows nul; PHPDocs should document this exception.
docs/​api/​app.md	Documents plain nul on Windows.

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