feat: add timelock contract for delayed admin actions

[bright5455]

closes #986

Summary

Adds a timelock Soroban contract that wraps privileged admin operations with a mandatory delay, letting the community observe and cancel actions before they execute.

Scope

• queue_action(action_type, params, eta) — enforces eta >= now + min_delay
• execute_action(action_id) — only after eta, otherwise TimeRemaining
• cancel_action(action_id) — admin only
• get_queued_actions() — returns all pending actions with eta and params
• Minimum delay configurable, defaults to 48 hours

Acceptance criteria

• execute_action blocked before eta → TimelockError::TimeRemaining
• Queue enforces min delay → TimelockError::MinDelayNotMet
• Cancel restricted to authorized admin
• get_queued_actions returns correct params + eta
• ActionQueued / ActionExecuted events emitted (plus ActionCancelled)

Tests

cargo test -p timelock → 9 passed, 0 failed. Clippy clean with -D warnings.

Files

• timelock/Cargo.toml (new)
• timelock/src/lib.rs (new, ~410 LOC incl. tests)
• timelock/test_snapshots/test/*.json (auto-generated, 9 files)
• Cargo.toml (add timelock to workspace members)
• Cargo.lock (cargo update)

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