Add tailnew: follow the newest log file in a directory

tailnew tails the most recently started file in a directory and switches
to each new one as it appears, for watching harnesses such as boxmaker's
run-plan.sh that write one log per task. A file counts as new when its
name is new, when the name points at a different file, or when it has
shrunk (a rerun truncating its old log).

Three front ends share one follower: plain tail -f style output, a
ratatui terminal view with scrollback, and an egui window with a folder
dialog and a Recursive checkbox. The Makefile provides build, gate and a
per-user install that also installs a desktop entry for the window.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tailnew/dist/tailnew.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=tailnew
+GenericName=Log Follower
+Comment=Follow the newest log file in a folder
+Exec=@BINDIR@/tailnew --gui %f
+Icon=utilities-log-viewer
+Terminal=false
+Categories=Development;
+Keywords=log;tail;follow;
+StartupWMClass=tailnew