[Feature] V2: testnet volume tests with concurrent multi-position disputes

[collinsezedike]

Summary

V1's scripts/testnet-load.sh exercises simple, single-party dispute volume. V2 needs its own load test that actually exercises many concurrent third-party positions on a single dispute, and multiple disputes running concurrently, since that's the scenario v2 adds that v1 never had.

What to build

A script (following the pattern of scripts/testnet-smoke.sh and scripts/testnet-load.sh) that, against a real testnet deployment:

• Opens several assertions and disputes them concurrently.
• For at least one dispute, funds many third-party positions (more than a handful, enough to meaningfully stress the incremental W-tracking and per-position storage from [Feature] V2: implement dispute and third-party registration #66) from distinct funded testnet identities.
• Drives at least one dispute to a strict-majority result and at least one to the optimistic timeout default, to exercise both settlement paths from [Feature] V2: implement settlement and forfeiture distribution #69.
• Confirms settlement and withdrawal work correctly regardless of the order callers process positions in, per the O(1)/order-independence invariant in [Feature] V2: implement settlement and forfeiture distribution #69.
• Records timing/storage observations useful for the economic simulation issue (see below): how registration/reveal/settlement behave under real network latency with many positions.

Depends on

#65-#70 (needs the full assert-through-withdraw lifecycle working).

[collinsezedike]

Implemented and merged in #91.