fix: commit header events atomically

[mcollina]

Make header semantic updates atomic with their event batch.

Previously, special-header state such as Content-Length and Transfer-Encoding was committed before both active header events were known to fit. If the name event fit but the value event did not, parsing returned without consuming the header while retaining its semantic effects; retrying the same bytes then failed as a duplicate.

The parser now reserves capacity for all active name/value range events, plus the error and terminator reserve, before validating or mutating any header state.

Mirrored Rust and WASM regressions fill the event buffer to the exact partial-fit boundary, verify that no framing state is committed, then retry and successfully complete both Content-Length and chunked messages. The full Rust/fuzz and 99-test WASM suites pass, with focused non-SIMD coverage.

This fixes SEC-07 from #38.