feat(notes): add line-level three-way merge util

Wraps node-diff3 to merge two edits derived from a common ancestor.
Conservative by design: edits to immediately-adjacent lines are
reported as conflicts so the caller can fall back to manual resolution
rather than risk a wrong auto-merge.

src/utils/threeWayMerge.spec.ts

@@ -0,0 +1,94 @@
+import { describe, expect, it } from "vitest"
+
+import { threeWayMerge } from "./threeWayMerge"
+
+const base = "line1\nline2\nline3\nline4\nline5"
+
+describe("threeWayMerge", () => {
+  it("merges non-overlapping edits cleanly, keeping both changes", () => {
+    const ours = "OURS1\nline2\nline3\nline4\nline5"
+    const theirs = "line1\nline2\nline3\nline4\nTHEIRS5"
+
+    const { clean, merged } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe("OURS1\nline2\nline3\nline4\nTHEIRS5")
+  })
+
+  it("treats identical edits on both sides as clean (false conflict)", () => {
+    const edited = "line1\nCHANGED\nline3\nline4\nline5"
+
+    const { clean, merged } = threeWayMerge(base, edited, edited)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(edited)
+  })
+
+  it("flags overlapping edits on the same line as not clean", () => {
+    const ours = "line1\nline2\nOURS3\nline4\nline5"
+    const theirs = "line1\nline2\nTHEIRS3\nline4\nline5"
+
+    const { clean } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(false)
+  })
+
+  it("returns the base unchanged when neither side edited", () => {
+    const { clean, merged } = threeWayMerge(base, base, base)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(base)
+  })
+
+  it("returns our edits when only we changed", () => {
+    const ours = "OURS1\nline2\nline3\nline4\nline5"
+
+    const { clean, merged } = threeWayMerge(base, ours, base)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(ours)
+  })
+
+  it("returns their edits when only they changed", () => {
+    const theirs = "line1\nline2\nline3\nline4\nTHEIRS5"
+
+    const { clean, merged } = threeWayMerge(base, base, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(theirs)
+  })
+
+  it("preserves a trailing newline", () => {
+    const baseNl = "a\nb\nc\n"
+    const ours = "A\nb\nc\n"
+    const theirs = "a\nb\nC\n"
+
+    const { clean, merged } = threeWayMerge(baseNl, ours, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe("A\nb\nC\n")
+  })
+
+  it("conservatively flags edits on immediately-adjacent lines as not clean", () => {
+    // No unchanged line separates the two edits, so node-diff3 groups them
+    // into one region and reports a conflict (git would merge this).
+    const ours = "A\nline2\nline3\nline4\nline5"
+    const theirs = "line1\nB\nline3\nline4\nline5"
+
+    const { clean } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(false)
+  })
+
+  it("merges added lines on different sides without conflict", () => {
+    const ours = "line1\nOURS-NEW\nline2\nline3\nline4\nline5"
+    const theirs = "line1\nline2\nline3\nline4\nline5\nTHEIRS-NEW"
+
+    const { clean, merged } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(
+      "line1\nOURS-NEW\nline2\nline3\nline4\nline5\nTHEIRS-NEW"
+    )
+  })
+})

src/utils/threeWayMerge.ts

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+import { diff3Merge } from "node-diff3"
+
+export interface ThreeWayMergeResult {
+  clean: boolean
+  merged: string
+}
+
+/**
+ * Line-level 3-way merge. `ours` and `theirs` are both derived from `base`
+ * (the common ancestor). When the two sets of edits are separated by at least
+ * one unchanged line they merge automatically (`clean: true`); when they
+ * overlap, `clean` is false and the caller should fall back to manual
+ * resolution.
+ *
+ * Conservative by design: node-diff3 groups *consecutive* changed lines into a
+ * single region, so edits to immediately-adjacent lines (no unchanged line
+ * between them) are reported as a conflict even when they don't truly overlap.
+ * This errs toward asking the user rather than risking a wrong auto-merge.
+ *
+ * `merged` is only meaningful when `clean` is true — conflicting regions are
+ * dropped from it, so it must not be committed on a non-clean merge.
+ */
+export const threeWayMerge = (
+  base: string,
+  ours: string,
+  theirs: string
+): ThreeWayMergeResult => {
+  // diff3Merge(a, o, b): a = ours, o = ancestor, b = theirs. The default
+  // excludeFalseConflicts folds identical edits made on both sides.
+  const regions = diff3Merge(ours, base, theirs, { stringSeparator: /\r?\n/ })
+
+  const lines: string[] = []
+  let clean = true
+  for (const region of regions) {
+    if (region.ok) lines.push(...region.ok)
+    else clean = false
+  }
+
+  return { clean, merged: lines.join("\n") }
+}