fix(v2): use checked arithmetic in register position and weight calculation (#131)

[s6pa1rta3n-lab]

Description

In tholos-v2, register computed a voter's new position amount and the assertion's updated eligible total weight using plain integer addition and subtraction:

let new_amount = previous_amount + amount;
...
let new_total = resolution.eligible_total - previous_amount + new_amount;

While values are checked against policy.max_position and policy.max_total_weight immediately after, plain arithmetic in release Wasm builds could silently wrap before reaching the bound checks.

Changes Made

• Replaced plain +/- operations in register with checked_add and checked_sub.
• Added Error::RegistrationArithmeticOverflow = 37 to pub enum Error.
• Documented RegistrationArithmeticOverflow in docs/src/CONTRACT_V2.md.
• Added unit tests in contracts/tholos-v2/src/test.rs:
  • test_register_position_amount_overflow_returns_error: validates that top-up position amount arithmetic overflow returns Error::RegistrationArithmeticOverflow.
  • test_register_eligible_total_overflow_returns_error: validates that eligible_total arithmetic overflow returns Error::RegistrationArithmeticOverflow.
• Generated and committed deterministic test snapshots.

Closes #131

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC

[collinsezedike]

@s6pa1rta3n-lab Closing — #131 is assigned to ZacLou, not you.