fix(s3): starting a member's workspace makes its missing folder, as the member

v2.7.3's ENOCWD left a new member no web path to a first project: their
home is no workspace (EFOLDER) and a not-yet-existing folder could not
start. run-as.makeMemberFolder: mkdir -p through write-as, AS the member,
only inside their own home (never the home itself, never a host folder,
never for a member with no account). Test: server-s3-lanes.

server/lib/run-as.js

@@ -488,6 +488,26 @@ function sameOwner(...paths) {
 
 /** `mkdir -p` / `rm -rf` of a path in a member's home, AS the member. */
 const mkdirAs = (acct, dir) => writeAs(acct, dir, "mkdir");
+
+/**
+ * S3: make a missing workspace folder inside a member's own home, AS them
+ * (`mkdir -p`, so only where they could themselves). A new member has no
+ * other way to their first project: their home is no workspace (EFOLDER).
+ * False — nothing made — for any other folder, the home itself, or a member
+ * with no usable account.
+ */
+function makeMemberFolder(dir) {
+  let acct;
+  try {
+    acct = accountForPath(dir);
+  } catch {
+    return false;
+  }
+  const abs = path.resolve(dir);
+  if (!acct || abs === acct.home) return false;
+  mkdirAs(acct, abs);
+  return true;
+}
 const removeAs = (acct, p) => writeAs(acct, p, "remove");
 
 /** `[cmd, argv]` for `command args` in `cwd`: as its member, else as is. */
@@ -528,6 +548,7 @@ module.exports = {
   handoffWrite,
   writeAs,
   mkdirAs,
+  makeMemberFolder,
   removeAs,
   launchArgv,
   sameOwner,