refactor: split lib.rs, add rustdoc and enforce 90% coverage

[egorsmkv]

Split the extension into api, config, checker, worker and stream modules; lib.rs now only registers the existing Python API. Document module responsibilities and functions, and validate rustdoc with warnings denied. Preserve proxy handling, timeouts, concurrency and body-read semantics.

Expand testing to 11 Rust tests and 8 Python API/integration tests. Cover malformed/unsupported proxies, HTTP 200/503 and both body modes, truncated bodies, transport failures, optional result fields and elapsed saturation, queue item/error/end messages and malformed messages, event-loop failures, worker initialization errors, empty streams and bounded concurrency. All network tests use local servers.

Add the CI check Coverage (>=90%). scripts/coverage.sh uses LLVM instrumentation across Rust unit tests and Python tests against the built wheel, then enforces --fail-under-lines 90. No production module is excluded; test files and dependencies are excluded. The job summary includes a progress bar, and rust-coverage artifacts include measured SVG/JSON/HTML reports. Move the PyO3 extension-module feature to Maturin so Rust tests can embed Python while wheels retain extension-module mode.

Validation: cargo clippy --locked --all-targets -- -D warnings, formatting/diff checks, rustdoc with -D warnings, 11 Rust tests, 8 Python tests, and the complete coverage script passed locally on CPython 3.12. Production Rust line coverage: 98.15% (319/325 lines); region coverage 93.35%. This threshold covers lines, not branch coverage. CI also retains Python 3.10/3.15 API jobs.