diff --git a/quc/src/diff.js b/quc/src/diff.js
new file mode 100644
index 0000000..9366081
--- /dev/null
+++ b/quc/src/diff.js
@@ -0,0 +1,74 @@
+// Line diffs for the chair view and the patch download. Myers' O(ND) algorithm, then unified-diff hunks.
+
+const lines = (s) => (s === "" ? [] : s.replace(/\r\n/g, "\n").replace(/\n$/, "").split("\n"));
+
+// Edit script between two line arrays: [{op: " " | "-" | "+", line}].
+export function diffLines(a, b) {
+  const n = a.length, m = b.length, max = n + m;
+  const v = new Int32Array(2 * max + 2), trace = [];
+  for (let d = 0; d <= max; d++) {
+    trace.push(v.slice());
+    for (let k = -d; k <= d; k += 2) {
+      let x = k === -d || (k !== d && v[max + k - 1] < v[max + k + 1]) ? v[max + k + 1] : v[max + k - 1] + 1;
+      let y = x - k;
+      while (x < n && y < m && a[x] === b[y]) { x++; y++; }
+      v[max + k] = x;
+      if (x >= n && y >= m) return backtrack(trace, a, b, max);
+    }
+  }
+  return [];
+}
+
+function backtrack(trace, a, b, max) {
+  const out = [];
+  let x = a.length, y = b.length;
+  for (let d = trace.length - 1; d >= 0; d--) {
+    const v = trace[d], k = x - y;
+    const prevK = k === -d || (k !== d && v[max + k - 1] < v[max + k + 1]) ? k + 1 : k - 1;
+    const prevX = v[max + prevK], prevY = prevX - prevK;
+    while (x > prevX && y > prevY) out.push({ op: " ", line: a[--x] }), y--;
+    if (d > 0) out.push(x === prevX ? { op: "+", line: b[--y] } : { op: "-", line: a[--x] });
+  }
+  return out.reverse();
+}
+
+// Unified diff of one file. Returns {text, added, removed}; text is "" when nothing changed.
+export function unified(path, before, after, context = 3) {
+  const a = lines(before ?? ""), b = lines(after ?? "");
+  const ops = diffLines(a, b);
+  let added = 0, removed = 0;
+  for (const o of ops) o.op === "+" ? added++ : o.op === "-" ? removed++ : 0;
+  if (!added && !removed) return { text: "", added, removed };
+  const hunks = [];
+  let i = 0, ai = 0, bi = 0;
+  while (i < ops.length) {
+    if (ops[i].op === " ") { i++; ai++; bi++; continue; }
+    const start = Math.max(0, i - context);
+    let end = i, j = i;
+    while (j < ops.length) {
+      if (ops[j].op !== " ") { end = j; j++; continue; }
+      let run = 0;
+      while (j + run < ops.length && ops[j + run].op === " ") run++;
+      if (j + run >= ops.length || run > 2 * context) break;
+      j += run;
+    }
+    const stop = Math.min(ops.length, end + 1 + context);
+    const lead = i - start;
+    let aStart = ai - lead, bStart = bi - lead, aLen = 0, bLen = 0;
+    const body = [];
+    for (let t = start; t < stop; t++) {
+      const o = ops[t];
+      body.push(o.op + o.line);
+      if (o.op !== "+") aLen++;
+      if (o.op !== "-") bLen++;
+    }
+    for (let t = i; t < stop; t++) {
+      if (ops[t].op !== "+") ai++;
+      if (ops[t].op !== "-") bi++;
+    }
+    hunks.push(`@@ -${aLen ? aStart + 1 : aStart},${aLen} +${bLen ? bStart + 1 : bStart},${bLen} @@\n${body.join("\n")}`);
+    i = stop;
+  }
+  const from = before == null ? "/dev/null" : `a/${path}`, to = after == null ? "/dev/null" : `b/${path}`;
+  return { text: `--- ${from}\n+++ ${to}\n${hunks.join("\n")}\n`, added, removed };
+}
diff --git a/quc/src/runs.js b/quc/src/runs.js
new file mode 100644
index 0000000..5d261ac
--- /dev/null
+++ b/quc/src/runs.js
@@ -0,0 +1,66 @@
+// Run model: queued -> running -> ended(finished|safeworded|unlocked), an append-only event log per run, one active run per visitor.
+// `store` is anything with async get(key) / put(key, string, opts?) / delete(key): a Workers KV namespace, or memoryStore() in tests.
+// Keys: run:<id> (record), events:<id> (array), work:<id> (repo text + edits), active:<visitor> (that visitor's queued/running run).
+// shortcut: KV has no compare-and-set and is eventually consistent across locations, so two simultaneous starts by one visitor can
+// both pass the check, and a reader far from the writer can lag a few seconds; move runs to a Durable Object when that matters.
+import { validate } from "./validate.js";
+
+export const KINDS = ["diff", "commit", "commentary", "unlock", "note", "system"];
+export const OUTCOMES = ["finished", "safeworded", "unlocked"];
+const NEXT = { queued: ["running", "ended"], running: ["ended"] };
+
+const fail = (code, message, extra) => Object.assign(new Error(message), { code }, extra);
+const load = async (s, k, empty = null) => JSON.parse((await s.get(k)) ?? "null") ?? empty;
+const save = (s, k, v) => s.put(k, JSON.stringify(v));
+
+export const getRun = (store, id) => load(store, `run:${id}`);
+export const putRun = (store, run) => save(store, `run:${run.id}`, run);
+export const getEvents = (store, id) => load(store, `events:${id}`, []);
+export const getWork = (store, id) => load(store, `work:${id}`, null);
+export const putWork = (store, id, work) => save(store, `work:${id}`, work);
+export const dropWork = (store, id) => store.delete(`work:${id}`);
+
+export function memoryStore() {
+  const m = new Map();
+  return { m, get: async (k) => m.get(k) ?? null, put: async (k, v) => void m.set(k, v), delete: async (k) => void m.delete(k) };
+}
+
+export async function createRun(store, { repo, task, visitor }) {
+  if (typeof visitor !== "string" || !visitor) throw fail("visitor", "visitor token required");
+  const errors = validate(repo, task);
+  if (Object.keys(errors).length) throw fail("invalid", "invalid run", { errors });
+  const active = await store.get(`active:${visitor}`);
+  if (active) {
+    const r = await getRun(store, active);
+    if (r && r.status !== "ended") throw fail("active", "you already have a run going", { id: active });
+  }
+  const run = { id: crypto.randomUUID().replace(/-/g, "").slice(0, 10), repo: repo.replace(/\.git\/?$|\/$/g, ""), task, status: "queued",
+    visitor, createdAt: new Date().toISOString(), level: 0, notes: [], vetoed: [], plan: null, queue: [], done: [], stats: { files: 0, added: 0, removed: 0 } };
+  await putRun(store, run);
+  await store.put(`active:${visitor}`, run.id);
+  return run;
+}
+
+export async function setStatus(store, run, status, outcome) {
+  if (!NEXT[run.status]?.includes(status)) throw fail("transition", `cannot go ${run.status} -> ${status}`);
+  if (status === "ended" && !OUTCOMES.includes(outcome)) throw fail("outcome", "ended needs an outcome: " + OUTCOMES.join("|"));
+  const at = new Date().toISOString();
+  run.status = status;
+  if (status === "running") run.startedAt = at;
+  else Object.assign(run, { endedAt: at, outcome });
+  await putRun(store, run);
+  if (status === "ended") await store.delete(`active:${run.visitor}`);
+  return run;
+}
+
+export async function addEvent(store, id, kind, data) {
+  if (!KINDS.includes(kind)) throw fail("kind", "event kind must be " + KINDS.join("|"));
+  const events = await getEvents(store, id);
+  const event = { seq: events.length + 1, at: new Date().toISOString(), kind, data };
+  events.push(event);
+  await save(store, `events:${id}`, events);
+  return event;
+}
+
+// The run as anyone may see it: no visitor token.
+export const publicRun = ({ visitor, ...rest }) => rest;
diff --git a/quc/src/validate.js b/quc/src/validate.js
new file mode 100644
index 0000000..9207af9
--- /dev/null
+++ b/quc/src/validate.js
@@ -0,0 +1,8 @@
+// Shared by the page (inlined via toString) and the server. Returns {repo?, task?} errors; {} is valid.
+export function validate(repo, task) {
+  const e = {};
+  if (!/^https:\/\/github\.com\/[\w.-]+\/[\w.-]+?(\.git)?\/?$/.test(repo)) e.repo = "Use a public GitHub repo URL, like https://github.com/owner/name.";
+  if (task.length < 10) e.task = "Say a bit more: at least 10 characters.";
+  else if (task.length > 200) e.task = "Keep it to one line: 200 characters at most.";
+  return e;
+}