feat: advantage and disadvantage dice rolls

[valb-mig]

Closes #18

Overview

Implements advantage and disadvantage mechanics for dice rolls, following the standard tabletop RPG convention: roll two dice of the same type and keep the highest (advantage) or lowest (disadvantage) result.

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What changed

RollAttribute enum

• Added RPGPlayground\Domain\Enums\Roll\RollAttribute with Advantage and Disadvantage cases
• Replaces the previous RollAttributes class with const string — now type-safe

RollDiceInput

• Added optional ?RollAttribute $attribute parameter to create()
• Defaults to null — no breaking change for existing callers

RollDiceUseCase

• Extracted private roll() method to handle dice collection
• When attribute is set, an extra roll is added and max() or min() is applied before modifiers
• Modifiers are still applied after advantage/disadvantage resolution

Tests

• Added advantage and disadvantage sections to RollDiceUseCaseTest
• Added attribute coverage to RollDiceInputTest
• D1 used as deterministic dice to guarantee assertions

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Behaviour

Attribute	Result
null	sum of all rolls
Advantage	highest of two rolls
Disadvantage	lowest of two rolls

Modifiers are always applied after attribute resolution, regardless of mode.