Resolve HOME to the target user's home directory instead of /root

ResolvedUser now carries home, looked up from the image's own
/etc/passwd entry for the final resolved uid (falling back to /root
for uid 0 or / otherwise when there's no matching row). bwrap's HOME
now uses this whenever a user override applies (--user/--group or an
image-declared default user); the plain /root default is kept only
when no override applies at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

src/user_spec.cpp

@@ -119,5 +119,15 @@ std::optional<ResolvedUser> resolve_user_and_group(const std::string& user,
         }
     }
 
-    return ResolvedUser{uid, gid};
+    // Looked up by the final resolved uid (field 2), independent of whether `user`
+    // was given as a name or a number, so it matches whichever /etc/passwd row
+    // actually owns that uid. No matching row -> fall back to "/root" for uid 0
+    // (matches useradd-less images' own convention for root) or "/" otherwise.
+    std::string home = uid == 0 ? "/root" : "/";
+    auto home_entry = lookup_entry(passwd_file, std::to_string(uid), 2);
+    if (home_entry && home_entry->size() > 5 && !(*home_entry)[5].empty()) {
+        home = (*home_entry)[5];
+    }
+
+    return ResolvedUser{uid, gid, home};
 }