feat: idle escrow yield integration — deposit to external yield protocol

[AJtheManager]

feat: idle escrow yield integration

lib.rs — 3 new public methods + 1 query:

• set_yield_protocol(admin, protocol) — admin configures the external yield contract address.
• deposit_idle_to_yield(admin, token, amount) — pushes idle escrow tokens to the yield protocol. Guards: amount ≤ (TotalEscrowed − AlreadyDeposited), paused-check, admin-only.
• withdraw_from_yield(admin, token) -> i128 — pulls principal + accrued yield back. Any yield above principal is credited to TotalEscrowed so it flows through normal milestone payouts.
• get_yield_deposited(token) -> i128 / get_yield_balance(token) -> i128 — read-only queries.

The external protocol interface is defined via #[contractclient(name = "YieldProtocolClient")] — matching the same pattern as token::Client — with three methods: deposit, withdraw, balance_of.

New DataKey variants: YieldProtocol, YieldDeposited(Address).

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test: fuzz testing for milestone percentage validation

property_tests.rs — new milestone_percent_fuzz module with 8 pure-model proptest cases (10 000 cases each) and a nested contract_percent_tests module with 4 contract-backed proptest cases (200 cases each):

Pure model: sum/min invariant, two-split validity, order independence, zero-entry rejection, >100 entry, augmented sum invalidation, uniform n-way split, empty list and exact-min-split edge cases.

Contract-backed: verifies the live contract accepts valid splits, rejects below-minimum percents, rejects non-100 sums, and accepts a single 100% milestone.

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feat: batch create multiple independent shipments

lib.rs — new BatchShipmentParams contracttype struct (mirrors all create_shipment params) and batch_create_shipments(env, params: Vec) -> Vec. Atomicity is free — Soroban transactions are all-or-nothing, so any per-shipment validation failure reverts the entire batch. Emits a batch_shipments_created event with the count.

closes #103
closes #88
closes #106
closes #101

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