fix: specify closure handler signature

[simonyang08]

Summary

• give the registry handler Closure alias an explicit variadic signature
• allow strict PHPStan users to decorate RegistryInterface without a missing callable signature error

Root cause

The Handler PHPStan alias used a bare \Closure, so consumers inheriting the alias received missingType.callable under strict analysis.

Current validation status

• consumer-only minimal PHPStan reproducer: fails before and passes with the proposed alias
• ReferenceHandlerTest: 4 tests, 9 assertions pass
• git diff --check: passes

Upstream CI exposed a variance conflict that the isolated reproducer missed: the proposed Closure(mixed...): mixed type rejects the repository's valid handlers with narrower required parameter lists. The full PHPStan job reports 29 argument.type errors introduced by this alias. The PR is therefore intentionally Draft while the correct way to represent an arbitrary Closure signature is discussed; the current commit should not be merged as-is.

Fixes #468