Pr 946 clean

[Killerjunior]

Summary

Closes #798

Created pr-946-clean from upstream/main and cherry-picked only the intended PR 946 commits:

• Added README.md
• Added crate description in Cargo.toml
• Wired crate docs in lib.rs
• Added regression test in test.rs

Notes

• The original branch included unrelated frontend/backend history.
• This clean branch contains only the actual PR changes.
• Dockerfile and soroban-event-worker.ts were not modified in this new branch.

Request

Please review this clean branch and merge once CI is green.

[ogazboiz]

verified the docs against the source: error codes 1-11 match errors.rs, all 11 event topics match events.rs, the fee-collection points and the rounding-to-zero example are accurate, and the lib.rs change is doc-only (include_str + one collect_fee doc comment). builds + tests pass. merging as the clean, contracts-only version of #946.

two tiny doc nits for a follow-up: the StreamError table omits code 12 StreamPaused, and InvalidRate is net_amount/duration (post-fee) not amount/duration.