Add scalar math, named dimensions, and Doctrine integration

[khaledalam]

Three utility features that round out the arithmetic and open a second framework ecosystem.

Added

• Scalar math on Quantity: times(), dividedBy() (guards zero), pow() (dimension-aware, (2 m)² = 4 m²), sqrt() (halves the dimension, throws on odd dimensions / negatives), abs(), negate().
• Named dimensions — Quantity::dimensionName() / Dimension::name() return "velocity", "force", "pressure", etc. (null when unrecognized). Plus Dimension::power() / Dimension::root().
• Doctrine integration — KhaledAlam\Unit\Doctrine\QuantityType (DBAL ^4) persists a Quantity as a string column; works in Symfony and any Doctrine project (parallels the Laravel cast).

Changed

• Dimension::__toString() emits unescaped Unicode so temperature reads as {"Θ":1}.

Verification

• 100% coverage maintained — 9/9 classes, 59/59 methods, 368/368 lines (PCOV).
• 94 tests, 367 assertions. PHPStan level max clean, CS-Fixer clean. New runnable examples/scalar-math.php.