Preserve pinned content across digest refolds

[coderabbitai]

Summary

Redesign durable pinned-message handling for digest refolds.

DigestRequest::pinned currently carries Vec<Sequence>. On a second or later fold, a pinned sequence can refer to a row that is already covered by ChannelDigest. The request contains only the unstructured prior digest and new rows in (after, through]. A stateless Digester therefore cannot recover the pinned row content and can omit it when it rewrites the digest.

Required process

Follow the repository convention for a payload-shape change:

1. Write or update a specification in docs/specs/.
2. Create a linked, implementation-ordered plan in docs/plans/.
3. Implement the redesign in a later change.

Required implementation scope

• Define a durable sequence-to-content representation for desk-visible pinned rows.
• Carry that representation through ChannelDigest and DigestRequest.
• Update digest-folding logic in crates/tinyhivemind/src/digest/mod.rs to preserve pinned material from both:
  • rows newly folded in the current request; and
  • rows already covered by the prior ChannelDigest.
• Update deterministic wire-shape tests for the changed payload.
• Document serialization, visibility, ordering, and privacy constraints.

Affected areas

• crates/tinyhivemind/src/digest/types.rs
• crates/tinyhivemind/src/digest/mod.rs
• Digest tests and deterministic wire-shape tests
• docs/specs/
• docs/plans/

Acceptance criteria

• A specification defines the durable pinned-content contract and its privacy constraints.
• An implementation plan orders the work from tests through API and serialization changes.
• A pin to a desk-visible row survives a first fold and a later refold.
• A later DigestRequest provides the digester enough content to retain pins that are no longer in messages.
• Private pinned rows do not leak into digest data.
• The wire representation is deterministic and covered by regression tests.

Backlinks

• Reported from PR Promote the utterance surface, fold by size, and back the desk example with a real provider layer #35: Promote the utterance surface, fold by size, and back the desk example with a real provider layer #35
• Review thread: Promote the utterance surface, fold by size, and back the desk example with a real provider layer #35 (comment)
• Requested by: @senamakel