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/README.md

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+# tailnew
+
+Follows the newest file in a directory, like `tail -f`, and switches to each new file as it
+appears. Built for watching harnesses such as boxmaker's `tools/run-plan.sh`, which write one log
+per task.
+
+    make install                       # ~/.local/bin/tailnew and a desktop entry for the window
+    tailnew .state/runs/M2a            # plain output, `==> file <==` banner on each switch
+    tailnew -t -r .state/runs          # full-screen terminal view, every milestone's directory
+    tailnew --gui -r .state/runs       # desktop window (egui)
+    tailnew --gui                      # desktop window, asks for the folder first
+
+`make gate` checks formatting, runs clippy with and without the window, runs the tests, and
+validates the desktop entry. `PREFIX=/usr/local make install` installs elsewhere;
+`cargo build --no-default-features` builds without the window.
+
+A file is new when its name has not been seen before, when the name now points at a different
+file, or when it has shrunk (a rerun that truncated its old log with `>`). The directory is polled
+(`-i MS`, default 250) and need not exist yet. Hidden files are ignored; `-g '*.log'` narrows the
+match further.
+
+Keys in the full-screen terminal view (`-t`):
+
+| key                     | action                                             |
+|-------------------------|----------------------------------------------------|
+| ↑ ↓ / k j               | scroll a line                                      |
+| PgUp PgDn / b space     | scroll a page (ctrl-u / ctrl-d: half a page)       |
+| g / G                   | top / back to following the tail                   |
+| [ ]                     | older / newer file (pins the view to it)           |
+| p                       | pin to the current file; again to unpin and jump to the newest |
+| L                       | unpin and jump to the newest file                  |
+| w                       | toggle line wrapping                               |
+| f                       | toggle the file list                               |
+| q / esc                 | quit                                               |
+
+The window (`--gui`) has a **Folder…** button (the desktop's folder dialog, also `o`) and a
+**Recursive** checkbox, buttons for the same file controls, a clickable file list, and the keys
+`[` `]` `p` `L` `f`, `G`/End to jump to the end, and `q`/Esc to quit. Started without a folder,
+as from the launcher, it opens the folder dialog straight away. It scrolls with the mouse,
+sticks to the bottom while you are there, and does not wrap long lines (scroll sideways instead).