feat(proposalModel): setProposalApplied re-anchors + stores original (#64)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/proposalModel.ts

@@ -37,6 +37,25 @@ export function removeProposal(artifact: Artifact, proposalId: string): boolean
   return artifact.proposals.length < before;
 }
 
+/**
+ * F12/#64 (INV-48): record a proposal as optimistically applied — store the
+ * pre-apply `original` (for revert + the struck `<del>`) and re-point its anchor
+ * fingerprint to the now-in-buffer applied text so `resolve()` finds it. Idempotent
+ * shape: a second call simply overwrites with the same values.
+ */
+export function setProposalApplied(
+  artifact: Artifact,
+  proposalId: string,
+  appliedFp: Fingerprint,
+  original: string,
+): boolean {
+  const p = artifact.proposals.find((x) => x.id === proposalId);
+  if (!p) return false;
+  p.original = original;
+  artifact.anchors[p.anchorId] = { fingerprint: appliedFp };
+  return true;
+}
+
 /** Markdown comment body: instruction header + fenced whole-range diff. */
 export function proposalBody(targetText: string, p: Proposal): string {
   const header = p.instruction ? `**Claude proposes** — _${p.instruction}_` : "**Claude proposes**";