feat(contracts): multisig event registry (#292) + event batch (#293) + DID resolver (#295) - #400

… (#293) + DID resolver (#295)

Three new Soroban contracts added under contracts/, each with its own
docs/<name>.md file.

#292 — Contract: Multi-Signature Registry for High-Value Event Emission
- New contract `multisig_event_registry`.
- M-of-N signer set configured at init; admin can override per-event-type
  threshold via `set_threshold`.
- `propose(event_type, topic2, data)` → returns proposal_id and records
  the proposer's vote; `approve(proposal_id)` auto-executes once the
  threshold is met, emitting both the underlying caller-requested event
  AND a `ms_attest` event with the full signer-approval list for
  non-repudiation. `cancel(proposal_id)` callable by proposer/admin.
- 7 tests cover: threshold-1 instant execution, threshold-2 with second
  approval, non-signer rejected, double-approve rejected, per-event
  threshold override, proposer cancellation, init validation.

#293 — Contract: Gas-Optimized Event Batching Utility Contract
- New contract `event_batch`.
- `emit_batch(sender, entries: Vec<EventEntry>) -> u32` emits up to
  MAX_BATCH_SIZE (64) events plus a single `batch_emt` summary in one
  invocation, amortizing dispatch + auth overhead.
- `EventEntry { topic, topic2, data: Bytes }` carries type-erased
  payloads so a single batch can mix event shapes.
- 5 tests cover: 2-event batch + summary, dual-topic entry, empty
  batch rejected, oversized batch rejected, full MAX_BATCH_SIZE batch.

#295 — Contract: Decentralized Identity (DID) Resolver on Soroban
- New contract `did_resolver`.
- W3C-DID-Core CRUD: register/update/resolve/revoke. DID documents
  stored as opaque `Bytes` keyed by `String` (intended to be the full
  `did:soroban:<id>` string). Single controller per DID.
- Monotonic `version` counter on update; revoked DIDs resolve to
  `Some(doc)` with `status = Revoked` rather than `None`, and are
  non-recyclable.
- Every state transition emits a `did_attst` event with `(did,
  controller, version)` — the event-based identity-verification hook
  called out in the issue's technical requirements.
- 9 tests cover: register+resolve, unknown returns None, double-register
  rejected, update bumps version + emits identity attestation, non-
  controller cannot update, revoke still resolves, cannot update after
  revoke, empty did rejected, multi-update identity attestation stream.

Workspace:
- All three crates added to contracts/Cargo.toml workspace members.
- 21/21 tests pass; release builds clean.
- Docs added under docs/ matching the existing per-contract doc style.