Cirreum.SignedRequest

RFC 9421 (HTTP Message Signatures) + RFC 9530 (Content-Digest) primitives for .NET: the shared signature-base builder, structured-field wire parser, Content-Digest compute/verify, the pluggable signing-algorithm seam (HMAC-SHA256 built in), and the outbound HttpRequestMessage/HttpClient signer. Pure and dependency-free — the common language shared by the Cirreum.Authentication.SignedRequest server scheme and the .Client SDK so a signed request verifies byte-identically on both sides (ADR-0021 §8).