fix(stellarkraal): prevent same-ledger loan ID collisions - #162

Summary

• add a checked contract-instance nonce to the open_loan ID seed so a same-asset reopen in the same ledger does not reuse the settled loan's storage key
• keep the public open_loan ABI unchanged and return ArithmeticError if the nonce cannot advance
• add an open → full repayment → reopen regression without ledger advancement, asserting distinct IDs and preservation of the original closed loan record
• update the affected Soroban snapshots

Validation

Negative control on base cca5f033318f43cb9b18060d161873d75df58381: failed as expected because both opens produced 183e3685794406772e06e088efceb884f8c4a88ea55d54d5fbb398d5429e7938.

Scoped checks on this change:

• focused same-ledger reopen regression — PASS
• stellarkraal tests — PASS (18 unit, 6 lifecycle, 0 doc tests)
• cargo fmt -p stellarkraal -- --check — PASS
• cargo clippy --locked -p stellarkraal --all-targets -- -D warnings — PASS
• cargo clippy --locked -p stellarkraal --lib -- -D warnings -D clippy::arithmetic_side_effects — PASS
• stellarkraal release WASM build — PASS

Repository-wide status

The scoped package checks above pass. Repository-wide gates remain blocked on current main by unrelated, pre-existing issues: the nested [workspace] in contracts/carbon_credit/fuzz, duplicate TTL constants in carbon_marketplace after a validation-only workspace bypass, and existing carbon_marketplace formatting drift. This PR does not modify those paths.

Fixes #94