Add explicit maximum length checks for packed custom share arrays

[starknetdev]

Summary

Packed custom share storage uses u8 slot indexes. With 15 shares per slot, the practical maximum is 3,840 shares. Larger arrays panic through try_into().unwrap() instead of failing with a clear validation error.

Affected Code

• packages/metagame/src/prize/prize_store.cairo:184
• packages/metagame/src/prize/prize_store.cairo:102
• packages/utilities/src/distribution/packed_shares.cairo:100
• packages/utilities/src/distribution/packed_shares.cairo:107

Risk

Configuration failure and poor error behavior for oversized custom distributions.

Suggested Fix

Define a max custom share count constant and reject larger arrays before storage loops.

Suggested Tests

Add boundary tests for:

• 3840 shares succeeds
• 3841 shares reverts with a clear error