#47 (SLICE-2, review): per-block document proposals + word-precise accept

Document-edit flow SLICE-2 (specs/coauthoring-document-edit-flow.md §7.2,
INV-39/40/41 — P1 "too much to review"). A whole-document rewrite now proposes
ONE F4 proposal per CHANGED BLOCK (the unit a human reviews), but accepting a
block reconciles attribution at WORD granularity (the unit F3 records). Block =
decision unit; word = attribution unit. Supersedes INV-37's per-word cut for
document edits; selection edits unchanged.

- trackChangesModel.ts: new pure diffToBlockHunks(current, rewritten) — block-key
  alignment (reusing diffArrays/diffBlocks keying): an isolated changed block →
  one block-aligned hunk → the rewritten block raw (a code/mermaid fence is one
  atomic whole-fence hunk, INV-23); insert/delete runs → one gap-span hunk over
  the inter-anchor region (separators included) so reconstruction stays exact; a
  zero-width gap-span is anchored (INV-41). Also split diffToHunks into the raw,
  un-anchored wordEditHunks + the anchoring wrapper (the anchoring could grow an
  insertion to overlap an adjacent hunk, corrupting a batch apply — a latent bug
  that only surfaced once hunks are applied as a batch).
- trackChangesPreview.ts: runEditAndPropose document branch uses diffToBlockHunks
  and tags each proposal granularity:"block".
- model.ts / proposalModel.ts: additive optional Proposal.granularity
  ("block"|"single"; absent ⇒ single, back-compat — no migration).
- proposalController.ts: accept of a block proposal runs an intra-block word
  sub-diff (wordEditHunks, disjoint) and applies one applyAgentEdit per changed
  run, descending offset — only the words Claude changed land Claude-attributed;
  unchanged spans keep prior authorship (INV-40).
- Tests: diffToBlockHunks unit (reconstruction + fence atomic + add/remove);
  f12Review host E2E (M blocks→M proposals, unchanged→none, fence atomic, INV-40
  attribution, INV-41 insertion accept); updated the f11 document-path E2E to
  per-block (INV-39 supersedes INV-37); MANUAL-SMOKE-F12 §2.

Seam note: pendingEdits.matchEvent resolves one registration per change event, so
INV-40's per-run attribution is sequential applyAgentEdit calls (N undo steps),
not one multi-replace WorkspaceEdit — see transcript Deferred decisions.

214 unit + 69/5 host E2E green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/proposalController.ts

@@ -18,7 +18,7 @@ import { addProposal, removeProposal } from "./proposalModel";
 import type { AttributionController } from "./attributionController";
 import type { VersionGuard } from "./versionGuard";
 import { isAuthorable } from "./workspacePath";
-import type { ProposalView } from "./trackChangesModel";
+import { wordEditHunks, type ProposalView } from "./trackChangesModel";
 
 /** Test-facing snapshot of what is currently rendered for a document. */
 export interface RenderedProposal {
@@ -122,7 +122,7 @@ export class ProposalController implements vscode.Disposable {
     fp: Fingerprint,
     replacement: string,
     author: Provenance,
-    opts?: { turnId?: string; instruction?: string },
+    opts?: { turnId?: string; instruction?: string; granularity?: "block" | "single" },
   ): Promise<string | undefined> {
     if (!this.isTracked(document)) return undefined;
     if (this.guard.isReadOnly(this.keyOf(document))) return undefined;
@@ -164,12 +164,19 @@ export class ProposalController implements vscode.Disposable {
       this.renderAll(document);
       return false;
     }
-    const range = new vscode.Range(document.positionAt(resolved.start), document.positionAt(resolved.end));
-    // No awaits between resolve and the seam call: document.version is current.
-    const ok = await this.attribution.applyAgentEdit(document, range, proposal.replacement, proposal.author, {
-      expectedVersion: document.version,
-      turnId: proposal.turnId,
-    });
+    // #47 (INV-40): a BLOCK proposal applies the whole block but attributes only
+    // the words Claude actually changed; a single proposal applies its whole range.
+    const ok =
+      proposal.granularity === "block"
+        ? await this.acceptBlock(document, resolved, proposal)
+        : await this.attribution.applyAgentEdit(
+            document,
+            new vscode.Range(document.positionAt(resolved.start), document.positionAt(resolved.end)),
+            proposal.replacement,
+            proposal.author,
+            // No awaits between resolve and the seam call: document.version is current.
+            { expectedVersion: document.version, turnId: proposal.turnId },
+          );
     if (!ok) {
       void vscode.window.showWarningMessage(
         "Cowriting: the editor rejected the accept — the proposal is still pending.",
@@ -181,6 +188,41 @@ export class ProposalController implements vscode.Disposable {
     return true;
   }
 
+  /**
+   * #47 (INV-40): accept a BLOCK proposal — apply Claude's whole block, but
+   * attribute ONLY the runs Claude actually changed. The block is the DECISION
+   * unit; the word is the ATTRIBUTION unit. An intra-block word sub-diff
+   * (`wordEditHunks`, the raw engine INV-37 used, repurposed — un-anchored so the
+   * runs stay disjoint for a batch apply) yields the changed runs; each lands
+   * through the F4 seam (Claude-attributed), applied last-position-first so an
+   * earlier run's offsets stay valid under the later ones. Unchanged spans within
+   * the block are never touched, so their prior authorship stands. (Each run is
+   * one seam edit / one undo step — see the spec's deferred note on undo grouping.)
+   */
+  private async acceptBlock(
+    document: vscode.TextDocument,
+    resolved: OffsetRange,
+    proposal: Proposal,
+  ): Promise<boolean> {
+    const blockText = document.getText(
+      new vscode.Range(document.positionAt(resolved.start), document.positionAt(resolved.end)),
+    );
+    const subHunks = wordEditHunks(blockText, proposal.replacement);
+    if (subHunks.length === 0) return true; // block already equals the proposal — nothing to attribute
+    for (const h of [...subHunks].sort((a, b) => b.start - a.start)) {
+      const range = new vscode.Range(
+        document.positionAt(resolved.start + h.start),
+        document.positionAt(resolved.start + h.end),
+      );
+      const ok = await this.attribution.applyAgentEdit(document, range, h.replacement, proposal.author, {
+        expectedVersion: document.version,
+        turnId: proposal.turnId,
+      });
+      if (!ok) return false;
+    }
+    return true;
+  }
+
   private reject(state: DocState, proposal: Proposal): void {
     if (this.guard.isReadOnly(state.docPath)) return;
     this.store.update(state.docPath, (a) => removeProposal(a, proposal.id));