feat: implement v2 reveal phase and commitment verification

[collinsezedike]

Summary

Implements #67: the reveal phase, where third-party positions disclose the side they committed to during registration.

• reveal(voter, id, choice, salt): looks up the voter's Position, verifies (choice, salt) against its stored commitment, and adds the position's weight to Resolution.agree_weight or disagree_weight.
• Commitment verification uses a VoteCommitmentPreimage struct hashed via ToXdr, the same canonical-encoding approach policy_hash already uses (see [Feature] V2: implement PolicySnapshot and versioned assertion state #64), rather than a hand-rolled byte concatenation. Matches V2_RESOLUTION.md's H(canonical_encode("THOLOS_V2_VOTE", network_id, contract_address, policy_hash, assertion_id, round, voter, choice, salt)) spec exactly, with round fixed at 0 (this design has exactly one weighted round).
• The Registration -> Reveal transition is lazy: the first reveal call after registration_deadline triggers it, matching V2_RESOLUTION.md's "a caller may advance an expired phase permissionlessly" principle rather than adding a separate entrypoint. That same transition auto-counts and auto-marks-revealed the asserter's and disputer's fixed positions, since their sides are already public and they never call reveal themselves; a Fixed voter calling reveal is naturally rejected with AlreadyRevealed since their position is already marked revealed at that point, no separate error variant needed.
• register's existing phase check already refuses new deposits once phase leaves Registration, so W freezes as a side effect of the transition with no extra code.
• New RevealOpened event for the phase transition itself, alongside Revealed per reveal.

One ordering fix worth calling out: reveal checks the assertion's phase before fetching its Resolution, not after. An uncontested assertion that reached Resolved via finalize never has a Resolution at all (only dispute creates one), so fetching it first would've surfaced a misleading AssertionNotFound instead of NotReveal for that case.

Closes #67

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo test --workspace --locked all pass (67 v1 + 60 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
• Commitment verification tested both for correct reveals (weight counted on the right side) and tampered ones (wrong salt, wrong choice)
• Fixed-position auto-reveal tested directly (weight counted, marked revealed, can't be revealed again)