feat: resolve issues #718, #719, #689, #690

[bade2brazy]

Summary

This PR resolves issues #718, #719, #689, and #690:

1. #718 – contracts/governor: emit DetailedVoteCast / vote event with voter rationale string or IPFS CID

• Added cast_vote(env, voter, proposal_id, support) and cast_vote_with_rationale(env, voter, proposal_id, support, rationale) to DripGovernor.
• Emits vote_cast event with (symbol_short!("vote"), voter, proposal_id) and data (support, rationale).
• Added unit tests in contracts/governor/src/tests.rs.

2. #719 – contracts/stream: support cliff percentage unlock (e.g. upfront unlock at cliff followed by linear stream)

• Added CliffConfig { cliff_time: u64, cliff_unlock_amount: i128 } to contracts/stream/src/storage.rs.
• Added set_cliff and cliff_config entry points to DripStream.
• Updated math::streamed_amount to credit cliff_unlock_amount immediately upon cliff_time expiration and linear streaming thereafter.
• Added unit tests in contracts/stream/src/tests.rs.

3. #689 – scripts/deploy.sh: support dry-run simulation mode before broadcasting transactions

• Added --dry-run flag to scripts/deploy.sh.
• Validates the existence of all 6 WASM artifacts, checks deployer identity, and simulates salt derivations without broadcasting transactions.

4. #690 – contracts/stream: add recipient / third-party top-up capability to allow external gift funding

• Updated DripStream::top_up to authenticate caller.require_auth() rather than restricting to sender/operator, allowing recipients and third-party benefactors to contribute additional funds.
• Emits standard topped_up event.
• Added unit test in contracts/stream/src/tests.rs.

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Closes #718
Closes #719
Closes #689
Closes #690

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