Raise test coverage to 100% and harden humanize()

[khaledalam]

Codecov was ~45% because coverage collection can't attribute autoload-time code (Name::label() and the bootstrap file ran during autoload, before the coverage window). This restructures so all logic is measured.

Changes

• Default registration extracted to a testable DefaultUnits::register(); bootstrap.php is now a one-line shim (excluded from coverage as an unmeasurable autoload entrypoint).
• New PHPUnit test suite (*Test.php) alongside the existing .phpt tests — covers the enum labels, registry, dimensions, unit stringification, Quantity edge branches, and the Laravel provider + cast. This also addresses the earlier "contributors expect PHPUnit, not .phpt" concern.
• humanize() bug fix — it previously called get() on every ladder symbol and threw if the registry was only partially populated; it now skips unregistered ladder units. Found by the new coverage test.

Result

• 100% coverage — Classes 8/8, Methods 44/44, Lines 296/296 (measured locally with PCOV).
• 56 tests, 299 assertions, no deprecations. PHPStan level max clean, CS-Fixer clean.