feat: escrow module — multisig approvals, time-lock & rate limiting (#288)

[A6dulmalik]

Closes #288

Summary

New contracts/escrow Soroban crate implementing the full escrow module:

• M-of-N multisig: configurable signer set (max 20) and approval threshold; admins can add/remove signers (with threshold auto-adjustment guards), change the threshold, time-lock delay, grace period, rate limit, and pause/unpause.
• Time-lock + grace period: queued transactions unlock after timelock_delay seconds and remain executable for grace_period; afterwards they expire and become cancellable by anyone (garbage collection).
• Multiple instances: create_escrow(token, is_native, purpose) mints independent purpose-specific escrow ids with per-instance balance accounting. Token escrows move SAC/ERC20-style tokens via TokenClient; native escrows account internally.
• Queue/cancel: signers or admins queue withdrawals (submitter counts as first approver); admin/submitter may cancel pre-execution.
• Rate limiting: sliding-window cap on executions both globally and per recipient, pruning old timestamps on each execution.
• AccessControl integration: optionally link an AccessControl contract at init; holders of its Admin role can administer escrow settings through a cross-contract has_role call.
• Events on every state transition.

Validation

• cargo clippy -p escrow --all-targets: 0 errors / 0 warnings
• cargo fmt applied
• 15 unit tests: init validation (thresholds, dupes, zero params), token+native deposits, M-of-N approve/revoke flows, time-lock boundaries, grace expiry + third-party cancel, cancel permissions, rate-limit saturation + window reset, pause gating, signer management bounds, AC-role delegation, close/deposit rejection, multi-instance isolation, view lookups

[devIKargi]

Weldone @A6dulmalik just run `Cargo fmt' and push