fix: harden mux lifecycle, framing, and backpressure

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Summary

• preserve accepted stream writes and enforce carrier-level flush plus PSH-before-FIN ordering
• make orderly connection close release the carrier, reject late work, avoid read/write waker races, and wake concurrent close callers safely
• enforce hard TX/RX queue bounds, include queued accepts and closed-stream payloads, and suspend keepalive expiry while RX is intentionally backpressured
• reject malformed control/reserved-ID frames without oversized decoder preallocation
• handle empty I/O correctly, reap idle accepts, report only open streams, and strengthen CI checks

Test-first coverage

Regression tests were added before each implementation fix and reproduced the failures around close, cancellation, flush, FIN ordering, queue limits, keepalive, malformed frames, empty I/O, and resource cleanup. The suite now contains 48 tests, including deterministic custom transports for shutdown and waker races.

Validation

• cargo test --all-features: 48 passed
• cargo test --lib --release: 48 passed
• cargo test --lib -- --test-threads=1: 48 passed
• cargo fmt --all -- --check
• cargo clippy --lib --tests --examples --all-features -- -D warnings
• RUSTDOCFLAGS=-D warnings cargo doc --no-deps --all-features
• git diff --check