feat: HTTP Trailers (RFC 9110 §6.5) across H1.1, H2, H3

[st0o0]

Summary

• H1.1 server trailer sending — chunked transfer encoding trailer section after final 0\r\n chunk (RFC 9112 §7.1.2). Kestrel does not support this — our differentiator.
• H3 server trailer sending — QPACK-encoded trailing HEADERS frame before QUIC FIN (RFC 9114 §4.1)
• H2 server bug fixes — buffered-body path bypassed trailer emission; client body reader hung when trailers arrived (DATA endStream:false → HEADERS END_STREAM)
• SetReadOnly freeze semantics — trailer dictionaries frozen before wire encoding across all protocols, matching Kestrel's pattern
• IHttpRequestTrailersFeature — server-side request trailer access with Available guard (throws before body consumed)
• Trailer header auto-generation — H1.1 auto-adds Trailer: response header listing field names when not explicitly set (RFC 9110 §6.6.2)
• Client-side verification — gap-fill tests confirming HttpResponseMessage.TrailingHeaders works across all protocols

Test plan

• 5866 unit+stage tests pass (0 failures)
• 111 E2E integration tests pass (H1.1: 4 trailer, H2: 3 trailer, H3: 2 trailer + existing)
• 474 client integration tests pass
• 89 server integration tests pass
• Total: 6540 tests, 0 failures
• H1.1 trailer round-trip: server sets trailers → client reads TrailingHeaders after body
• H2 trailer round-trip: gRPC status/message trailers pass through
• H3 trailer round-trip: QPACK-encoded trailers before QUIC FIN
• Prohibited fields filtered by TrailerFieldValidator
• All-prohibited trailers → no stream hang (H2 fallback to endStream:true)