[Contract] Emit structured events for all state transitions

[Gbangbolaoluwagbemiga]

Overview

Off-chain indexers and frontend real-time updates depend on contract events. Currently the contract emits no structured events — it's impossible to reliably stream escrow state changes.

Events to add

Event	Trigger
`EscrowCreated { escrow_id, depositor, total_amount, token }`	`create_escrow`
`WorkStarted { escrow_id, freelancer }`	`start_work`
`MilestoneSubmitted { escrow_id, index }`	`submit_milestone`
`MilestoneApproved { escrow_id, index, amount_released }`	`approve_milestone`
`MilestoneRejected { escrow_id, index, reason }`	`reject_milestone`
`DisputeRaised { escrow_id, index, raised_by }`	dispute path
`DisputeResolved { escrow_id, index, freelancer_amount }`	admin resolve
`EscrowCompleted { escrow_id }`	final approval
`EscrowRefunded { escrow_id, amount }`	refund paths
`FreelancerRated { escrow_id, rating }`	`rate_freelancer`

Acceptance criteria

• All 10 events emitted correctly using `env.events().publish()`
• Event structs use `contracttype` derive (ABI-exported)
• Unit tests assert events emitted via `env.events().all()`

Good first issue: Start with just `EscrowCreated` and open a draft PR.