feat(http/unstable): add RFC 9530 digest fields

[tomas-zijdemans]

Adds an unstable @std/http/unstable-digest-fields module for creating and verifying RFC 9530 Content-Digest and Repr-Digest field values.

RFC 9530, an IETF standards-track RFC from Feb 2024, replaces the legacy RFC 3230 Digest field with Structured Fields based digest headers for HTTP content and representation integrity. These helpers support creating digest values from strings, bytes, or streams, and verifying Request / Response bodies without consuming the original body.

This is also useful with HTTP Message Signatures (RFC 9421), where a signed digest field can bind the signature to the message body. The implementation is small and browser-compatible, reusing existing @std primitives: timing-safe-equal, unstable-structured-fields, and to-bytes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.62%. Comparing base (cd03740) to head (b0e16b6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7037      +/-   ##
==========================================
+ Coverage   94.61%   94.62%   +0.01%     
==========================================
  Files         634      635       +1     
  Lines       51799    51944     +145     
  Branches     9329     9361      +32     
==========================================
+ Hits        49009    49154     +145     
  Misses       2216     2216              
  Partials      574      574              

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[tomas-zijdemans]

Updated to use the stable toBytes

[tomas-zijdemans]

Parsing has been hardened and docs improved. Ready for review @bartlomieju