[CONTRACT-68] Add code coverage reporting for the contracts workspace

[mftee]

Overview

No coverage tool (cargo-tarpaulin, cargo-llvm-cov, or equivalent) or CI reporting step exists for contracts/, despite the workspace already having a reasonably strong existing test suite across all five crates — coverage tooling would make it possible to see where that strong baseline actually has gaps, rather than guessing (as several issues in this batch, like [CONTRACT-50], had to do by manual inspection).

Tasks

• Add cargo-llvm-cov (or cargo-tarpaulin) as a CI step for the contracts workspace.
• Publish a coverage report/badge and set a reasonable initial floor reflecting current coverage, to prevent regression rather than mandate an immediate jump.

Acceptance Criteria

• Coverage is measured and visible per PR, with a floor CI enforces.

Notes for Contributors

Requires Rust + Soroban. Workspace: contracts/ (members: identity, shipment, escrow, document, reputation). Run cargo test --all, cargo fmt --all -- --check, and cargo clippy --all-targets --all-features -- -D warnings before opening a PR — CI enforces all three. Comment below to be assigned.