refactor: extract shared admin and TTL utilities into a new mergefi-c…

[ABEEGOLD]

Closes #60

Extract extend_ttl and require_admin into mergefi-common crate (#60)

Overview

This PR resolves #60 by addressing the structural triplication of extend_ttl and require_admin across the mergefi-escrow, mergefi-milestones, and mergefi-maintenance-pool contracts.

By extracting these heavily duplicated, identical components into a shared mergefi-common dependency, this PR sets up the critical structural prerequisite for applying the TTL extension bounds and admin-rotation fixes securely, exactly once, across all contracts rather than risking incomplete application via copy-pasting.

Changes Made

• Created mergefi-common: Added a new shared Rust crate to the workspace specifically for cross-contract logic.
• Abstracted Common Helpers: Moved extend_ttl and require_admin into the common crate.
• AdminKey Trait Parameterization: Designed a minimal mergefi_common::AdminKey trait and implemented it for each contract's distinct DataKey enum, mapping their respective Admin variants seamlessly.
• Refactored Contract Implementations: Updated the lib.rs for escrow, milestones, and maintenance-pool to use the parameterized mergefi-common implementations. The original wrapper functions are kept as transparent pass-throughs to minimize diff noise and cleanly integrate without breaking local error mapping.
• Cargo Updates: Registered mergefi-common in the workspace root and injected it as a dependency for all three contracts.

Verification

As mandated by the "proof of behavioral equivalence" requirements:

• ✅ Zero Behavioral Drift: All 35 existing unit tests across the three contracts pass unmodified (cargo test --workspace).
• ✅ API Equivalence: No contract-facing functions or invocation paths were altered. The extraction works natively with existing contract state.

Note on Sequencing

This PR provides the groundwork for the upcoming batch of security fixes (instance-storage-TTL, admin/treasury-rotation, and #11's fixed-threshold). Please merge this extraction first, so the subsequent fixes only have to be written against the shared mergefi-common implementations rather than duplicated three times.

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[chonilius]

Good one