XML comments cite spec section '§5.1' which does not exist (spec has flat §5)

[nficano]

Category: docs Severity: minor
Location: src/Arcp.Core/Envelope/Envelope.cs:59-60

What

ARCP v1.1 §5 (Wire Format) has no subsection §5.1, yet the wire layer repeatedly cites '§5.1' — Envelope.cs:9 and :59, EnvelopeJsonConverter.cs:11/:104/:106, and Identifiers.cs:8. The correct citation is §5.

Evidence

    /// <summary>Unknown top-level envelope fields preserved verbatim per spec §5.1 ("MUST ignore
    /// unknown fields") so they round-trip without loss.</summary>

Proposed fix

Replace the §5.1 references with §5 across the Envelope/converter/identifier XML comments.

Acceptance criteria

• No source comment cites a non-existent spec section; wire-format references point to §5.

[nficano]

Fixed in #77 (merged).