feat: implement v2 PolicySnapshot and versioned assertion state

[collinsezedike]

Summary

New crate contracts/tholos-v2, a wholly separate contract from v1 (never upgraded in place, blue/green migration per V2_RESOLUTION.md). This PR implements #64's scope only: the immutable PolicySnapshotV2 pinned at assertion creation, and the AssertionV2 record it lives on.

• initialize: pins deployment-wide defaults (token, base_bond, registration/reveal durations, anti-sniping params, position/weight bounds) into a PolicySnapshotV2, validated and stored once.
• Decided parameters implemented: min_resolution_bond always equals base_bond (a cheaper minimum would let a third party break an asserter/disputer tie for a fraction of what they risked); bounded anti-sniping extension capped by a hard maximum.
• AssertionV2 stores the pinned policy, its hash, phase, and (once resolved) terminal_cause/final_outcome directly on the assertion, not only in events.
• get_policy / get_assertion: read-only lookups.
• create_pending_assertion: private helper building/storing a Pending AssertionV2 with a freshly pinned policy, not yet wired to a public entrypoint ([Feature] V2: implement bonded assertion posting #65 adds the real, bond-transferring assert_outcome on top of it) or moving any tokens. Marked #[allow(dead_code)] with a comment pointing at [Feature] V2: implement bonded assertion posting #65, which removes the allow.

One real SDK finding worth flagging for later v2 issues: soroban-sdk 26.1.0's contracttype derive doesn't generate an XDR ScVal conversion for Option<EnumType> (only Option of built-in types like Address/bool work), so AssertionV2 failed to compile with terminal_cause: Option<TerminalCause>. Worked around with a TerminalCause::NotYetDecided sentinel variant instead of None, documented inline. Worth remembering for #66-#71, which will define more enums.

Also fixed a real bug caught by testing, not by design: initialize's validation order made InvalidMaxTotalWeight's zero-case structurally unreachable (max_position <= max_total_weight always failed first when max_total_weight was 0). Reordered so total-weight bounds are checked before the position-relative check.

Closes #64

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo test --workspace --locked all pass (67 v1 + 21 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