Add actionId vocabulary to CanonicalDiff (back-compat with diffId)

[chubes4]

Summary

Data Machine unified its preview primitive on a generic "pending action" model (DM #1171, editor #5). Any tool invocation can now stage → preview → resolve through a single /actions/resolve endpoint and a single action_id — not just the three content-diff tools (edit_post_blocks / replace_post_blocks / insert_content) that originally drove the v0.7 diff primitive in this package.

Server-side CanonicalDiffPreview::build() now emits actionId on the diff payload. This package was only reading diffId / diff_id, so:

• parseCanonicalDiff returns diffId: '' on every fresh preview payload.
• DiffCard's onAccept / onReject callbacks fire with an empty string.
• Every consumer that wires those callbacks to a resolve endpoint silently breaks.

Fix

• Add actionId as the canonical field on CanonicalDiffData.

• Keep diffId as a deprecated alias, always populated with the same value as actionId, so consumers still reading diff.diffId keep working through one more major version.

• parseCanonicalDiff reads, in priority order:

  1. rawDiff.actionId
  2. rawDiff.action_id
  3. container.action_id
  4. rawDiff.diffId
  5. rawDiff.diff_id
  6. container.diff_id

  The resolved string populates both actionId and diffId on the returned CanonicalDiffData.

• DiffCard's onAccept / onReject now fire with diff.actionId || diff.diffId so the right id gets delivered regardless of which field the consumer wrote.

• Docblock example updated to show the new /actions/resolve + action_id shape.

Compat

• Additive only. No existing consumer breaks.
• Consumers reading diff.diffId keep working.
• Consumers receiving the onAccept(id) / onReject(id) callback keep working — they now get the right id even when the backend sent actionId.
• Consumers should migrate to diff.actionId on their schedule. The diffId alias can be removed in a future major.

Validation

• npm run build (tsc) passes cleanly.
• No test suite in the repo; behaviour verified by reading each call site for diffId / action_id / diff_id.

Downstream

• data-machine-frontend-chat has a companion PR that updates its resolve call to /actions/resolve with action_id and bumps the chat dep — depends on this PR.
• Data Machine admin ChatSidebar.jsx only uses copyChatAsMarkdown / useChat, not the diff primitives, so no change needed there.

AI assistance

• AI assistance: Yes
• Tool(s): Claude Code (Opus 4.7)
• Used for: Traced the vocabulary drift from DM's unified preview primitive (PR #1171) through CanonicalDiffPreview::build() to this package's parser, drafted the additive alias design, and verified no other internal call sites reference diffId. Chris flagged the audit target.