Make Fbp::$randomNumber private and add the missing native types

[loevgaard]

Summary

The last two cleanups from the list in #16 and #24. After this, nothing in src/ is untyped, and the cookie value objects are fully immutable.

• Fbp::$randomNumber is private. It was the only public, mutable property on the otherwise immutable Fb/Fbc/Fbp value objects, so $fbp->randomNumber = 1 bypassed withRandomNumber(). The getter and the wither already existed.
• User::$fbc and User::$fbp are natively typed, as Fbc|string|null and Fbp|string|null. They were untyped with only a docblock, so anything could be assigned.
• Fb::withCreationTime() takes int|\DateTimeInterface natively, instead of accepting anything and checking it with an assertion at runtime.
• withSubdomainIndex(), withCreationTime() and withAppendix() declare static as their return type, replacing the @return static docblocks.
• The private Parameters::normalize() helper gets mixed and a native return type.

Backwards compatibility

Intentional breaks, documented in a new section of UPGRADE-2.0.md:

• Reading or writing $fbp->randomNumber no longer works; use getRandomNumber() / withRandomNumber().
• Assigning anything but an Fbc/Fbp, a string or null to User::$fbc / $fbp throws a \TypeError.
• Passing a wrong type to withCreationTime() throws a \TypeError, where it used to throw an InvalidArgumentException. The upgrade guide notes that a \TypeError is PHP's own error for a programming mistake and does not implement ExceptionInterface.

The Roave BC job is red by design.

Tests

• Removed FbTest::it_rejects_a_creation_time_that_is_neither_an_integer_nor_a_datetime. It passed a string with a @phpstan-ignore, and only made sense for an untyped parameter; the type system enforces this now.
• Added UserTest::it_accepts_the_cookies_as_value_objects_and_as_strings: both properties default to null, and accept value objects as well as strings, with the expected payload each time.
• 166 tests, 100 % line coverage; PHPStan (level max), ECS, dependency analyser green; Infection MSI 95 % / covered 95 %, no new escaped mutants.

Not in this PR

Issue #20, one failing pixel stopping delivery to the rest, is still open. It was deferred on purpose and needs design decisions of its own.