feat(contracts): add emergency migration flow for paused rounds

[Ova-Klik]

Closes #1047

Summary

This PR implements an emergency migration path for funding rounds that become paused or otherwise unable to complete due to operational edge cases.

The implementation introduces a secure, permissioned recovery mechanism that enables authorized operators to migrate affected rounds while preserving contributor balances, maintaining contract invariants, and providing a complete on-chain audit trail.

Motivation

In exceptional situations, a funding round may become paused or enter a state where participant funds cannot progress through the normal lifecycle.

Without a recovery mechanism, contributors may be left with stranded funds despite no fault of their own.

This change introduces a controlled migration process that safely transitions eligible rounds while maintaining the protocol's safety guarantees and preserving user funds.

Changes

Emergency Migration

• Added a permissioned emergency migration entry point.
• Restricted migration execution to authorized administrators/governance.
• Prevented unauthorized migration attempts.
• Ensured migrations can only execute from valid emergency states.

State Management

• Support migration of paused rounds.
• Support migration of partially completed rounds.
• Preserve contributor balances and accounting throughout migration.
• Prevent duplicate or replayed migrations.
• Maintain valid contract state transitions.

Events

Added explicit contract events for:

• Emergency migration initiated
• Emergency migration completed
• Authorized executor
• Source and destination round information
• Migration metadata for auditability

These events improve observability and provide a transparent record of all emergency actions.

Safety

• Preserve contributor ownership and balance guarantees.
• Validate migration eligibility before execution.
• Reject invalid or inconsistent state transitions.
• Ensure migrations are deterministic and auditable.

Testing

Added tests covering:

• Successful emergency migration
• Migration from paused rounds
• Migration from partially completed rounds
• Unauthorized migration attempts
• Invalid migration states
• Duplicate migration protection
• Event emission
• Contributor balance preservation after migration

Documentation

Updated contract documentation to include:

• Emergency migration workflow
• Authorization requirements
• Supported recovery scenarios
• State transition expectations
• Event reference

Impact

This change affects only emergency recovery scenarios.

Normal round creation, contribution, settlement, withdrawals, and reward distribution continue to behave as before.

Checklist

• Permissioned migration implemented
• Contributor safety preserved
• Auditable events added
• Tests added
• Documentation updated
• cargo fmt passes
• cargo clippy -- -D warnings passes
• Contract tests pass

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[Ova-Klik]

@Cedarich kindly review pr, issue resolved. thanks

[Cedarich]

@Ova-Klik kindly fix workflow

[Cedarich]

@Ova-Klik