feat: implement v2 bonded assertion posting and uncontested finalize

[collinsezedike]

Summary

Implements #65: v2's assert_outcome equivalent (the optimistic first stage, before any dispute exists) and the uncontested-finalize path. Re-reading the issue body, the paragraph about finalize behavior turned out to be a real requirement, not just context: no later v2 issue (#66-#71) covers the uncontested case at all, so leaving it out here would mean nothing in the whole v2 chain ever returns an uncontested asserter's bond.

That also surfaced a real gap in #64: there was no field distinct from registration_duration_secs for "how long can a Pending assertion be disputed before it's eligible for finalize." Added:

• PolicySnapshotV2.challenge_window_secs and finalize_reward_bps (carried over from v1 unchanged: same incentive problem, same solution, independently justified since it's the identical uncontested case in both versions).
• AssertionV2.opened_at and finalizer.
• assert_outcome(asserter, outcome) -> u64: requires asserter auth, writes state before the external token transfer (matching v1's reentrancy-safe ordering), transfers base_bond, emits Asserted.
• finalize(caller, id) -> bool: requires caller auth unconditionally (same v1 hardening, so finalizer can never be spoofed regardless of reward), checks challenge_window_secs elapsed, splits the bond by finalize_reward_bps if configured, emits Finalized.
• MAX_BOND_AMOUNT is now properly derived (i128::MAX / MAX_FINALIZE_REWARD_BPS) instead of [Feature] V2: implement PolicySnapshot and versioned assertion state #64's interim placeholder, since this issue introduces the real reward-multiply arithmetic that bound protects.

Closes #65

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo test --workspace --locked all pass (67 v1 + 30 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
• Token balance assertions confirm the bond actually moves (asserter to contract on assert, contract to asserter/caller split correctly on finalize)