[FEATURE]: Add roll number interval overlap validator for multi-set assignments

[Hanu2908]

Description

In ClassHub, Class Representatives (CRs) can dispatch multi-set assignments where different question sets are mapped to roll number ranges (e.g., Set A: Rolls 1–25, Set B: Rolls 26–50).

Currently, if a CR accidentally inputs overlapping ranges (e.g., Set A: 1–25 and Set B: 20–45) or inverted boundaries (e.g., 25–1), the error is only caught on database insertion. We need a shared client-side interval validation function with immediate UI feedback in the CR Command assignment composer.

Relevant files

• src/lib/utils/rolls.ts (Roll parsing and number utilities)
• src/pages/app/CRCommandPage.tsx (Assignment creation modal & sets dispatcher)
• tests/unit/rolls.test.ts (Unit test suite for roll helper functions)

Requirements and acceptance criteria

1. Validation helper function:

   • Implement and export validateRollIntervals(sets: Array<{ rollStart: number; rollEnd: number; label?: string }>) in src/lib/utils/rolls.ts.
   • Return a structured result:

     interface IntervalValidationResult {
       isValid: boolean;
       errors: Array<{
         index: number;
         field: 'rollStart' | 'rollEnd' | 'range';
         message: string;
       }>;
     }

2. Validation rules:

   • Contiguous/Touching intervals: [1, 25] and [26, 50] are VALID.
   • Overlapping intervals: [1, 25] and [25, 50] (touching endpoints sharing roll 25) or [1, 25] and [20, 30] are INVALID ("Roll range overlaps with Set X").
   • Inverted boundaries: rollStart > rollEnd (e.g. 25 to 10) is INVALID ("Start roll must be less than or equal to end roll").
   • Non-positive values: rollStart <= 0 or rollEnd <= 0 are INVALID ("Roll numbers must be positive integers").

3. UI Integration:

   • In src/pages/app/CRCommandPage.tsx, run validateRollIntervals on change and display an inline error badge/message next to invalid set inputs.
   • Disable the "Dispatch Assignment" button while range validation errors exist.

4. Testing:

   • Add unit tests in tests/unit/rolls.test.ts covering non-overlapping sets, overlapping sets, subsets, inverted ranges, and single-student ranges (rollStart === rollEnd).
   • All tests must pass with npm test.

Testing locally

npm test tests/unit/rolls.test.ts