feat(tabs): place new, child and web tabs into groups

New sessions, spawned child sessions and saved web tabs now land in tab
groups instead of always starting in Ungrouped.

New session in a group:
- The group menu gains "New session" (first item). It expands a collapsed
  group and runs ONE launch with the current case and run mode; a launch
  already in flight is reported, not queued.
- run({ count, tabGroupId }) threads the group through every launch path
  (local POST /api/sessions, remote/docker and external-CLI
  /api/quick-start). The count overrides the steppers for that launch only.
- Server: POST /api/sessions and /api/quick-start accept an optional
  tabGroupId (Zod, 1-100 chars; null/empty is a 400). The routes pass it to
  addSession(), and TabLayoutService.sessionCreated() places the session at
  the end of that group in the SAME versioned write that first records it.
  The session is added to the live map inside the owner lock (with a
  rollback when the write fails), so no queued layout edit can commit it
  first. The group is looked up in the creating owner's own layout, so an
  unknown, deleted or foreign id is ignored instead of failing the spawn. An
  explicit group is a hand placement: a child placed this way is manual.
- The create response carries the committed tabLayout (only when tabGroupId
  was sent), adopted before the new tab is drawn. Responses without the
  field are unchanged.

Child sessions (parentSessionId / X-Codeman-Parent-Session):
- A child nobody placed follows its parent: the server's existing
  normalization already puts it after the parent's rows; the browser now
  draws a not-yet-stored child there too (project({ parents })) and, while
  groups exist, adopts the server's order instead of echoing its own append
  back through PUT /api/session-order (which pinned every child manual).
- The row menu says where a child sits (follows, placed by hand, parent
  closed, parent loop) and offers "Follow parent again" only to a hand-placed
  child. A child whose parent is gone stands on its own and is never
  re-adopted.

Saved web tabs:
- Web tabs are layout refs like sessions; openness stays per device. A
  grouped web tab shows (and takes an Alt+N slot) only while open here and
  reopens where the layout keeps it. Pinned by tests; no behaviour change.

Unchanged: the flat rail and header strip (markup, HTML5 drag, append and
save on create), the tree semantics and single tab stop, sessionOrder and
Alt+N, and every request that does not send tabGroupId.

src/tab-layout-service.ts

@@ -11,6 +11,7 @@ import { applyLegacySessionRank, recomposeGlobalSessionOrder } from './tab-layou
 import {
   flattenOwnerSessionOrder,
   materializeOrphans,
+  moveRef,
   normalizeTabLayout,
   TabLayoutValidationError,
   validateTabLayout,
@@ -67,6 +68,18 @@ export interface RemovedTabLayoutSession {
   owner?: string;
 }
 
+/** Where a newly created session asked to be placed. Advisory: an unknown group is ignored. */
+export interface SessionPlacementHint {
+  tabGroupId?: string;
+}
+
+/**
+ * Adds the created session to the live map. Runs INSIDE the owner lock, so no
+ * other layout mutation can observe (and commit) the session first; the returned
+ * function undoes it when the layout write fails.
+ */
+export type RegisterCreatedSession = () => void | (() => void);
+
 interface PreparedOwnerLayout {
   current: TabLayout | null;
   authoritative: TabLayout;
@@ -97,6 +110,25 @@ const sameLayout = (a: TabLayout, b: TabLayout): boolean => JSON.stringify(a) ==
 const sameOrder = (a: readonly string[], b: readonly string[]): boolean =>
   a.length === b.length && a.every((id, index) => id === b[index]);
 
+/** Move a just-created session to the end of `groupId`, or leave the layout alone. */
+function placeCreatedSession(
+  layout: TabLayout,
+  sessionId: string,
+  groupId: string,
+  metadata: readonly TabRefMetadata[]
+): TabLayout {
+  const group = layout.groups.find((candidate) => candidate.id === groupId);
+  if (!group) return layout;
+  const index = group.refs.filter((ref) => ref.kind !== 'session' || ref.id !== sessionId).length;
+  try {
+    return moveRef(layout, { kind: 'session', id: sessionId }, { groupId, index }, metadata);
+  } catch (error) {
+    // Not an owner-valid ref of this layout: the hint does not apply.
+    if (error instanceof TabLayoutValidationError) return layout;
+    throw error;
+  }
+}
+
 export class TabLayoutService {
   private restorationState: 'pending' | 'complete' | 'failed' | 'skipped' = 'pending';
   private readonly ownerQueues = new Map<string, Promise<void>>();
@@ -320,9 +352,13 @@ export class TabLayoutService {
     };
   }
 
-  private async getUnlocked(owner: string): Promise<TabLayout> {
+  private async getUnlocked(
+    owner: string,
+    adjust?: (layout: TabLayout, metadata: readonly TabRefMetadata[]) => TabLayout
+  ): Promise<TabLayout> {
     const prepared = await this.prepareUnlocked(owner);
-    if (!prepared.needsReconciliationCommit) {
+    const next = adjust ? adjust(prepared.authoritative, prepared.metadata) : prepared.authoritative;
+    if (!prepared.needsReconciliationCommit && sameLayout(next, prepared.authoritative)) {
       const publication = {
         owner,
         previous: prepared.current,
@@ -335,7 +371,7 @@ export class TabLayoutService {
       return prepared.authoritative;
     }
     const base = prepared.current ?? { ...prepared.authoritative, version: -1 };
-    return this.commit(owner, base, prepared.authoritative, prepared.metadata);
+    return this.commit(owner, base, next, prepared.metadata, prepared.current);
   }
 
   async get(owner: string): Promise<TabLayout> {
@@ -442,9 +478,37 @@ export class TabLayoutService {
     }
   }
 
-  /** Reconcile one completed session creation into one versioned mutation. */
-  async sessionCreated(owner: string): Promise<TabLayout> {
-    return this.get(owner);
+  /**
+   * Reconcile one completed session creation into one versioned mutation.
+   *
+   * `placement.tabGroupId` puts the new session at the end of that group in the
+   * same write. The group is looked up in THIS owner's layout only, so a group id
+   * from another owner (or one deleted meanwhile) is simply not found and the
+   * session keeps its normal placement: after its parent when it has one,
+   * otherwise at the end of Ungrouped. An explicit group is a hand placement, so
+   * a child session placed this way is marked `manual` and stops following.
+   */
+  async sessionCreated(
+    owner: string,
+    sessionId?: string,
+    placement: SessionPlacementHint = {},
+    register?: RegisterCreatedSession
+  ): Promise<TabLayout> {
+    return this.withOwner(owner, async () => {
+      const rollback = register?.();
+      try {
+        const groupId = placement.tabGroupId;
+        return await this.getUnlocked(
+          owner,
+          sessionId && groupId
+            ? (layout, metadata) => placeCreatedSession(layout, sessionId, groupId, metadata)
+            : undefined
+        );
+      } catch (error) {
+        if (typeof rollback === 'function') rollback();
+        throw error;
+      }
+    });
   }
 
   /** Reconcile one completed saved-webview creation into one versioned mutation. */