id: implement chat ↔ talk-page unification extensions in pkgs/id (from design #5872) #6222

Summary

Implementation follow-on for the design decision recorded on #5872 (chat ↔ talk-page unification). Filed so the design has a linked implementation track, per programme practice. Do not start until both gates below clear.

Gates (both required before work starts)

1. Timing decision — id: design: chat ↔ talk-page unification (from #5682) #5872's decision record restates the parent position: implementation timing is a separate decision, possibly months out. This issue waits for that decision.
2. Sibling designs landed — the anchor part-selector grammar and drift/re-location rules are id: design: chat anchor data model (from #5682) #5870's decision; talk-page lifecycle/permissions are id: design: talk-page semantics (from #5682) #5871's decision. This implementation consumes those decisions; it does not invent them.

Scope (per the #5872 decision record)

In pkgs/id (v1 chat lives in src/chat.rs: chat.message.v1 signed events in a channel group):

• Add chat.anchor.v1 events: bind/unbind/re-locate a conversation↔object anchor (link-type vocabulary from id: design: chat anchor data model (from #5682) #5870), part anchors by reference (object ID + part selector), never embedded content or capabilities.
• Add the conversation-level talk_for binding: a conversation designated as an object's one canonical talk page; first valid binding wins, conflicts fold to a visible disputed state.
• Extend ChatLink with a third kind, object (addressable ID object by ID + author), alongside v1's blob/event — this is the in-message anchoring and paste-a-link upgrade path.
• Retroactive anchoring: additive events only, referencing v1 messages by event ID; client link-recognition may propose anchors, a signed anchor event from a member is what makes one real.
• Projection changes must satisfy the eight invariants in the id: design: chat ↔ talk-page unification (from #5682) #5872 decision record (single log, additivity, no history mutation, deterministic fold, one canonical talk binding, no capability leakage, unknown-kind tolerance in older clients, orphan safety on object tombstone).

Acceptance

• All eight id: design: chat ↔ talk-page unification (from #5682) #5872 invariants covered by tests (PBT where the fold/projection invariants allow: additivity and determinism are property-testable over generated event logs).
• A v1-only conversation with no anchor events projects byte-identically to before this change.
• Two replicas folding the same log with a conflicting talk_for binding converge on the same disputed state.

Relations: design #5872; parent #5682; protocol #2036; chat v1 #2051 (closed, implemented); epic #2056.

Acceptance criteria

• The chat <-> talk-page unification extensions from the originating design are implemented in pkgs/id.
• New behavior is covered by tests at the id layer (anchor/binding events and link objects).
• Existing chat and talk-page behavior is unchanged for items that do not use the extensions.