Fix SplObjectStorage deprecations on PHP 8.5

[Metabor]

PHP 8.5 deprecates SplObjectStorage::attach() and ::detach(). Fifteen call sites in src/ trigger them, so every consumer running on PHP 8.5 gets deprecation notices coming out of this library.

What changes

attach($o) → offsetSet($o, null) and detach($o) → offsetUnset($o) — the replacements named in the deprecation message.

File	Lines
Metabor/Callback/Composite.php	30, 35
Metabor/Event/Event.php	51, 56
Metabor/KeyValue/Composite.php	19, 24
Metabor/Observer/Subject.php	20, 25
Metabor/Statemachine/State.php	39
Metabor/Statemachine/Factory/Factory.php	44, 49
Metabor/Statemachine/Graph/GraphBuilder.php	91, 99
Metabor/Statemachine/Observer/SubjectHasUpdated.php	15, 17

Plus the four occurrences in tests/.

Three attach() calls in src/ are deliberately left alone. They target Event and Statemachine, which define their own attach() — not SplObjectStorage: SetupHelper.php:111, StateCollectionMerger.php:149, Factory.php:66. A blind search-and-replace would have broken the public API here.

Equivalence

attach($o) stores null as its $info by default, and offsetSet($o, null) does the same — identical in behaviour, including return value, repeated insertion, count(), contains() and iteration. $info is never read anywhere in this repository.

Verification

The test suite is not sufficient evidence here, and that is worth spelling out: it only reports deprecations on the code paths it executes. Factory::attachStatemachineObserver, Factory::detachStatemachineObserver, Event::detach, KeyValue\Composite::detach, GraphBuilder and one branch of SubjectHasUpdated are reached by none of the 88 tests. A fix driven by the suite output stops at eight of the fifteen — and the suite then reports success.

Three independent checks on PHP 8.5.4:

1. Static — independent of any execution

$ grep -rn -- "->attach(\|->detach(" src/ | grep -v "public function"
3 hits, all three the intentional Event/Statemachine calls listed above

2. The uncovered paths, driven by hand

A script calling Factory::attachStatemachineObserver, Factory::detachStatemachineObserver and Event::detach — none of which any test reaches:

master:      4 deprecations  (Factory.php:44, Factory.php:49, Event.php:51, Event.php:56)
this branch: 0

3. The suite, for what it is worth

before:  Tests: 88, Assertions: 189, Deprecations: 12
after:   OK (88 tests, 189 assertions)

Behaviour

Running Statemachine-Example against the patched library produces byte-identical output (76 lines, 3630 bytes), and drops it from 21 deprecations per run to 0. Those 21 only surface under error_reporting=E_ALL; the stock php.ini hides them, which is part of why they went unnoticed.

Not addressed here

SubjectHasUpdated extends \SplObjectStorage, so the class inherits the deprecated attach()/detach() into its own public API. Both internal calls are fixed, but the inheritance is a design question, not a deprecation fix.