Add upgrade, pause, and role separation safeguards

[0xsamuel1]

Summary

• add separate admin/oracle roles across escrow, milestones, and maintenance-pool, including oracle/admin rotation helpers
• add admin-gated pause/unpause and upgrade entrypoints, storage version tracking, and paused-state guards for risky flows
• document Soroban upgrade/storage migration, emergency pause operations, two-key role separation, and maintenance-pool deposit history tradeoffs
• refresh README ABI snippets, backend operational guidance, and invoke/example scripts for the new initialize shape

Design Notes

• Routine payout paths now use the oracle key: escrow release, milestone release_issue, and maintenance-pool withdraw.
• High-trust operations remain admin-gated: pause/unpause, upgrade, role rotation, treasury rotation, and recovery paths.
• Pausing blocks new funding/deposits and oracle payout paths while keeping refund/recovery paths available where the contract supports them.
• Existing deployed testnet contracts cannot be retrofitted in place because they lack an upgrade entrypoint; the recommendation is a final redeploy with these safeguards before mainnet.
• Maintenance-pool pagination is documented as a spike outcome: keep the one-at-a-time getter for now, prefer event indexing for backend history, and revisit pagination after events land.

Tests

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo build --target wasm32v1-none --release -p mergefi-escrow -p mergefi-milestones -p mergefi-maintenance-pool

Closes #15.
Closes #14.
Closes #10.
Closes #13.

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