feat(escrow): add material/labor tranche split with release_materials()

[AlonsoFi]

Escrow Tranching: Material & Labor Split with Instant Material Release

Overview

This PR implements smart contract escrow tranching as specified in issue #163. It splits the escrow funding into two
distinct tranches — materials and labor — so artisans can receive materials funding immediately upon job acceptance
while the labor portion remains locked until work is complete.

Closes #163

Changes

🔗 Smart Contract (Soroban)

Updated Escrow struct: Added material_amount, labor_amount, and materials_released fields to support split funding.

Updated initialize(): Signature changed from a single amount to (material_amount, labor_amount). The total is computed
internally so the client still deposits in one transaction.

New release_materials(): Transfers material_amount to the artisan immediately, keeping the escrow in Funded state with
labor_amount still locked.

Updated release(): When materials were pre-released, only transfers the remaining labor_amount to the artisan.

Updated reclaim() and arbitrate(): Both functions operate only on the locked portion — respecting any prior material
release.

New MaterialsReleasedEvent: Emitted upon successful material tranche release.

🧪 Testing

6 new unit tests added:

• test_split_deposit_single_sign — client funds both tranches with a single deposit
• test_release_materials_basic — artisan receives material_amount; labor_amount stays locked
• test_release_materials_then_labor — full tranche workflow end-to-end
• test_release_materials_wrong_state — guard against releasing on non-Funded escrow
• test_release_materials_twice — double-release protection
• test_reclaim_after_materials_released — only labor_amount returned after deadline

All existing tests updated to match the new initialize() signature.