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/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "tailnew"
+version = "0.1.0"
+edition = "2024"
+description = "Follow the newest file in a directory, switching to each new file as it appears"
+
+[dependencies]
+clap = { version = "4.6", features = ["derive"] }
+eframe = { version = "0.32", optional = true }
+rfd = { version = "0.15", optional = true }
+ratatui = "0.29"
+unicode-width = "0.2"
+
+[features]
+default = ["gui"]
+# The desktop window (--gui) and its folder dialog. Build with --no-default-features for a terminal-only binary.
+gui = ["dep:eframe", "dep:rfd"]
+
+[dev-dependencies]
+tempfile = "3"