feat: add SSE search result statistics

[digizeph]

Summary

• Add final SSE result statistics for completed, cancelled, timeout, and error outcomes.
• Report matched elements, source-byte metadata, result rate, and matching source files.
• Update the monocle search --remote-url client for the new final-result payload.

Validation

• cargo fmt -- --check
• cargo test --all-features
• cargo clippy --lib --all-features -- -D warnings
• Manual monocle server → monocle search --remote-url SSE check

Fixes #135