feat: implement v2 dispute and third-party registration

[collinsezedike]

Summary

Implements #66: dispute() and the third-party registration phase that follows it.

• New Position record (one address's stake, keyed by (assertion_id, address)) and Resolution record (per-dispute deadlines and the running eligible total W), matching V2_RESOLUTION.md's storage layout: AssertionV2 stays claim/parties/policy, Resolution is the mutable per-dispute state that grows as reveal ([Feature] V2: implement reveal phase and commitment verification #67) and settlement ([Feature] V2: implement settlement and forfeiture distribution #69) land.
• dispute(disputer, id): rejects disputing your own assertion, creates the two initial Fixed positions (asserter for, disputer against), opens registration with a deadline, transfers the disputer's bond, emits Disputed.
• register(voter, id, amount, commitment): third-party deposits, salted-commitment-hidden side, aggregating top-ups that can't change their original commitment, atomic rejection if position/total bounds would be exceeded, bounded anti-sniping extension on late deposits.

Two SDK constraints found along the way, both now documented inline for later issues:

• #[contracttype] enum variants can't have named fields (only unit or single-unnamed-field tuple variants), so PositionKind::Fixed { agrees_with_asserter: bool } had to become PositionKind::Fixed(bool). Same general category as the Option<EnumType> gap [Feature] V2: implement PolicySnapshot and versioned assertion state #64 found.
• Also fixed a real gap in [Feature] V2: implement PolicySnapshot and versioned assertion state #64's initialize validation: nothing required anti_snipe_hard_max_secs >= registration_duration_secs, so a misconfigured deployment could have the hard deadline fall before the ordinary soft deadline. Added that check.

Closes #66

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo test --workspace --locked all pass (67 v1 + 49 v2 + 2 integration-example tests)
• cargo build -p tholos-v2 --target wasm32v1-none --release --locked succeeds
• mdbook build succeeds
• Every new Error variant has a dedicated test triggering it
• Anti-sniping extension tested both for triggering and for capping at the hard deadline (repeated late deposits from fresh addresses)
• Token balance assertions confirm bonds and deposits actually move