feat: add persistent storage TTL bumps for commitments

[Dayz-tech-co]

Closes #468

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#468
This PR adds persistent-storage TTL management for escrow commitments so active commitment records and owner indexes remain readable through their full maturity window.

What changed

• Added maturity-based TTL bump helpers in contracts/escrow/src/lib.rs

  • converts remaining commitment lifetime into Soroban ledgers
  • adds a small post-maturity buffer
  • caps TTL requests at the network max TTL

• Updated persistent write paths

  • save now bumps DataKey::Commitment(id) after each create/mutation write
  • index_owner recomputes the latest maturity in an owner’s commitment list and bumps DataKey::OwnerIndex(owner)
  • deindex_owner refreshes the remaining owner index TTL after ownership changes

• Added focused contract tests in contracts/escrow/src/test.rs

  • verifies commitment and owner-index TTLs are bumped on create
  • verifies a later mutation refreshes TTL when storage falls behind the remaining maturity horizon
  • verifies owner-index TTL tracks the latest commitment maturity

• Documented the strategy in contracts/README.md

  • explains maturity-based TTL targeting
  • explains the thresholded bump behavior and fee tradeoff

Why

The escrow contract stores commitments in persistent Soroban storage, but without explicit TTL extension long-duration commitments can expire before they mature. This PR keeps active entries alive for the full escrow duration while avoiding unnecessary rent-extension fees.

Validation

Ran:

• cargo check --lib --manifest-path contracts/Cargo.toml
• cargo test ttl_ --manifest-path contracts/Cargo.toml

Result:

• library compile passed
• new TTL-focused tests passed

Notes

• TTL bumps are conditional, not unconditional, so writes only pay extension fees when the current entry TTL no longer covers the remaining maturity horizon.
• The target TTL includes a small post-maturity buffer so release/refund can still execute after maturity.

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