feat(fanout): worktree fan-out board — race N agent lanes of one repo (W5)

Fan ONE prompt across N branch/agent lanes: N worktrees, N Claude sessions on the
same prompt, watched side-by-side in the split-grid board, approve/kill per lane,
🏆 keep the winner (losers' worktrees removed). ~90% composition of shipped parts.

- src/http/session-groups.ts (new, pure, no git exec): deriveRepoRoot /
  groupSessionsByRepo (cluster sessions by their <repo>-worktrees parent).
- GET /live-sessions/grouped (read-only; registered BEFORE /live-sessions/:id so
  "grouped" isn't captured as an id). MAX_FANOUT_LANES env (default 6, = grid-6 cap).
- public/fanout.ts (new, pure): buildFanoutCmd shell-quotes the prompt (single-quote
  wrap with '\''-escaping so $(...)/backticks/;/&& can't execute) + collapses newlines
  + caps 4000 chars; laneBranch/slugify. Effective N = min(lanes, maxFanoutLanes, 6),
  ≥2; the maxSessions cap is enforced server-side ("Started K of N" banner).
- public/tabs.ts launchFanout (N× createWorktree → openProject w/ prompt pre-injected
  via the existing initialInput) + keepFanoutWinner; public/projects.ts renderFanoutForm
  + extracted shared createWorktreeReq (DRY). Byte-shuttle preserved (lane = own PTY).
  One-click merge deferred (winner session stays open for a manual merge).

Reused unchanged: createWorktree/removeWorktree, addEntry/initialInput, split-grid +
per-quadrant approve/maximize/monitor + gauges. Verified: typecheck + build:web clean,
2063 pass at --test-timeout=30000 (only the known tmux/PTY flake red). Fixed 3
/config/ui exact-shape tests to include the new maxFanoutLanes field.

public/style.css

@@ -456,6 +456,20 @@ body {
   color: var(--text);
   background: var(--surface-3);
 }
+.cell-keep {
+  border: 0;
+  background: transparent;
+  color: var(--text-faint);
+  cursor: pointer;
+  font-size: 13px;
+  line-height: 1;
+  padding: 2px 4px;
+  border-radius: 5px;
+}
+.cell-keep:hover {
+  color: var(--text);
+  background: var(--surface-3);
+}
 .cell-monitor-btn {
   border: 0;
   background: transparent;
@@ -1944,6 +1958,60 @@ body {
   text-transform: uppercase;
   color: var(--text-faint);
 }
+/* W5 fan-out form — sibling of the New Worktree form. */
+.proj-fanout-form {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+.proj-fanout-prompt {
+  width: 100%;
+  box-sizing: border-box;
+  resize: vertical;
+  min-height: 44px;
+  font: inherit;
+}
+.proj-fanout-row {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+  flex-wrap: wrap;
+}
+.proj-fanout-lanes {
+  width: 72px;
+}
+.proj-fanout-branch {
+  flex: 1 1 160px;
+  min-width: 120px;
+}
+.proj-fanout-error {
+  color: var(--danger, #e5534b);
+  font-size: 12px;
+}
+.proj-fanout-submit {
+  align-self: flex-start;
+}
+.proj-fanout-submit:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+/* W5 fan-out status/error banner — fixed, dismissible, textContent only. */
+.fanout-banner {
+  position: fixed;
+  left: 50%;
+  bottom: 16px;
+  transform: translateX(-50%);
+  max-width: min(680px, 92vw);
+  z-index: 60;
+  padding: 10px 14px;
+  border-radius: 8px;
+  background: var(--surface-3, #2a2c33);
+  color: var(--text, #e7e8ec);
+  border: 1px solid var(--border, #3a3d46);
+  box-shadow: 0 6px 24px rgba(0, 0, 0, 0.35);
+  font-size: 13px;
+  cursor: pointer;
+}
 /* Section header with an inline action (CLAUDE.md generate/update). */
 .proj-section-row {
   display: flex;