Track lifetime settled amounts per agent

[jamilahmadzai]

Summary

• Add lifetime settled-value counters for each agent and for the full protocol.
• Update both settle and settle_all to add positive billed stroops with saturating arithmetic.
• Add read entrypoints, regression tests, README notes, and storage/API reference docs.

Closes #82

Bounty: GrantFox OSS campaign for issue #82

Security notes

• Counters are monotonic: non-positive bills leave settled totals unchanged, and positive bills use saturating_add to clamp at i128::MAX instead of overflowing.
• The existing settled event payload is unchanged.
• settle_all is included so batch drains cannot undercount lifetime settled value compared with single-service settle.
• New getters default to 0 for fresh agents/deployments.

Validation

• cargo fmt --all -- --check
• cargo build
• cargo test -p escrow total_settled -- --nocapture (5 passed)
• GitHub CI currently fails in the clippy step with the same existing escrow test-module warnings present on main (262c66b, latest main CI run 28393140941 also failed with could not compile escrow (lib test) due to 155 previous errors).
• cargo test currently fails on existing baseline failures reproducible from clean main, including the owner-scoped settlement expectation tests and event-count tests unrelated to this change.