feat: guard Deployer singleton before Host construction

[m1rm]

• Bug fix #…?
• New feature?
• BC breaks? -> maybe, because the exception changed
• Tests added?
• Docs added? -> comments in the code

Background

Host::__construct used Deployer::get() to decide whether to inherit the global recipe config. With a typed, uninitialized static $instance, PHP 8+ throws when get() reads the property before new Deployer(...) has run. That can break HostTest (and any code path that builds a Host without a bootstrapped singleton).

Solution

Deployer:

• Store the singleton as ?self (default null). get() throws a clear RuntimeException if not initialized.
• Add hasInstance() so callers can check without touching an uninitialized property.
• Add resetInstance() (@internal) for tests that need a clean singleton between cases.

Host:

• Use Deployer::hasInstance() before calling get() when wiring the parent Configuration.

HostTest:

• tearDown() calls Deployer::resetInstance() so tests stay isolated.
• Import Deployer\Deployer for tearDown.

Behavior / compatibility

• Normal runtime (always new Deployer before heavy use): unchanged.
• Calling Deployer::get() with no instance: still invalid; error is now RuntimeException instead of the engine error on an uninitialized typed static.

[m1rm]

Additional info/motivation of this PR: I came across the issue when implementing setDefaultSelector as thin typed wrapper for set('default_selector', ...) (which will be a subsequent PR).