feat(v2): add admin rotation

[dimin4241-svg]

Summary

Adds the same direct, current-admin-authorized set_admin mechanism to Tholos v2. Rotation takes effect immediately, emits AdminUpdated, and changes only who may invoke existing admin controls; pinned assertion policy and economics are unaffected. The v2 administration design documentation is updated.

Closes #129

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test pass locally (make check)
• CONTRACT.md updated if the public interface changed — not applicable because it documents v1; V2_RESOLUTION.md is updated instead
• Fresh v2 WASM was built, deployed to testnet, and initialized from this branch
• Manually rotated the fresh deployment, confirmed the old admin was rejected by set_paused_v2, and confirmed the new admin was accepted

[collinsezedike]

set_admin extends instance-storage TTL, but the existing set_paused_v2 doesn't. If set_admin is the only admin-initiated instance-storage-touching call over a long stretch, the instance entry still risks TTL expiration since set_paused_v2 never extends it. Consider fixing this at the shared level (a helper both functions call) rather than only in the function this PR adds.

[collinsezedike]

This note explains set_admin's semantics, but docs/src/DEPLOYMENT.md's separate Admin runbook section (Pausing during an incident, Rotating the resolver committee) has no equivalent step-by-step example for rotating the admin key. An operator following that runbook during an incident has no set_admin invocation example to copy, unlike pause and resolver rotation. Please add one there too.

[dimin4241-svg]

Addressed both review points in commit fec3633:

• set_paused_v2 now shares the instance-TTL renewal helper with initialize/set_admin, with a regression test covering both admin state-changing calls near expiry.
• Added a concrete set_admin invocation to the Admin runbook and clarified that the handoff is immediate.

cargo test -p tholos-v2 passes: 112 tests.

[collinsezedike]

@dimin4241-svg thank you for the contribution. This looks good, there is nothing to flag.