feat(model): add Proposal.original for F12 optimistic-apply revert (#64)

src/model.ts

@@ -97,6 +97,14 @@ export interface Proposal {
    * back-compat with older sidecars) ⇒ a single-range proposal accepted whole.
    */
   granularity?: "block" | "single";
+  /**
+   * F12/#64 (INV-48): the pre-apply text, captured when a proposal is
+   * optimistically applied to the buffer. `replacement` is now in the buffer and
+   * `fp.text` re-anchors to it, so `original` is the only record of what to revert
+   * to (revert-in-place) and what to show struck in the `<del>` half. Absent on a
+   * proposal created but not yet optimistically applied (or older sidecars).
+   */
+  original?: string;
 }
 
 export interface Artifact {
@@ -252,6 +260,8 @@ export function serializeArtifact(a: Artifact): string {
             createdAt: p.createdAt,
             ...(p.turnId !== undefined ? { turnId: p.turnId } : {}),
             ...(p.instruction !== undefined ? { instruction: p.instruction } : {}),
+            ...(p.original !== undefined ? { original: p.original } : {}),
+            ...(p.granularity !== undefined ? { granularity: p.granularity } : {}),
           },
           p,
         ),