Channel struct missing pinned_messages: HashSet<EventHash> field

[intendednull]

Problem

The Channel struct in crates/channel/src/lib.rs:245-270 does not have a pinned_messages field. The DAG spec requires:

pub struct Channel {
    // ...
    pub pinned_messages: HashSet<EventHash>,
}

The current struct only has:

pub id: ChannelId,
pub name: String,
pub topic: Option<String>,
pub kind: ChannelKind,
pub created_at: DateTime<Utc>,

Note: willow-state has its own Channel type in crates/state/src/types.rs that correctly includes pinned_messages: HashSet<EventHash>. The issue is that the willow-channel crate's Channel type is out of sync with the state crate's version.

Impact

If any code uses the willow-channel::Channel type instead of willow-state::Channel, pinned message tracking is lost.

Suggested fix

Either:

• Add pinned_messages: HashSet<EventHash> to willow-channel::Channel
• Consolidate to a single Channel type (remove duplication between crates)

Location

• crates/channel/src/lib.rs:245-270
• Compare with crates/state/src/types.rs (correct version)

References

• Spec: docs/specs/2026-04-01-per-author-merkle-dag-state-design.md (Section 2, ChatMessage Type Changes)
• Found during codebase audit against DAG spec