fix(http2): preserve empty header continuation sequences

[hatoo]

An HTTP/2 response can start with a HEADERS frame containing no HPACK bytes and finish in CONTINUATION frames. The client used a nonempty buffer as its continuation state, so it rejected these valid responses. The same gap allowed an interleaved DATA END_STREAM to count an invalid response as a success.

Track the unfinished field block with its existing stream ID, clear that state at END_HEADERS, and reject HEADERS on stream zero. Preserve the original END_STREAM flag and reuse the header buffer without adding connection fields or allocations. Regression tests cover empty, padding-only and priority-only fragments; empty continuations and trailers; every transport cut; wrong streams, interleaving and EOF; TCP/TLS request accounting and reconnects.

Validation:

• Five new parser regressions and three worker regressions fail on the unchanged baseline and pass after the fix.
• Debug, release, Rust 1.91 and musl: 460 unit tests and 62 integration tests pass per profile; two existing ignored tests remain. Formatting and Clippy pass.
• Independent nghttp2 1.59.0: 481 valid transport splits accepted and 728 invalid interleaving splits rejected. hyper-h2 4.3.0 agrees on four fragment variants; its padding-only limitation was checked separately with nghttp2 and RFC 9113 §6.2.
• Immutable release clients against fixed hfast, three alternating pairs per workload: 13,565,952 requests including warmups, zero errors. RPS paired medians: c1/p1 −5.13% (pairs −9.86%, −5.13%, −0.55%); c16/p64 −0.05% (−0.87%, +0.43%, −0.05%). WSL timing remains uncertain, particularly the adverse single-connection result. This PR makes no speedup or performance-equivalence claim.

HTTP/2 field-block sequencing rules. Required hosted CI includes debug/release tests and the existing Docker interoperability suite.