docs(terminal): say that the tail-depth signals have no consumer yet

`tail_full` and `tail_drained` are the queue bound, and nothing outside the
tests calls them: the runtime reader pumps `drain` to completion after every
read, so the tail cannot reach the cap and nobody needs to ask. That is fine
and it is the design -- the signal was built for the reader that stops
pumping -- but an exported, documented predicate with an empty call graph
reads as wired to anyone who doesn't go looking. Stated in the doc comment
instead, because an unused signal that looks connected is worse than one that
says it isn't.

Co-authored-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>
Signed-off-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>

desktop/src-tauri/crates/buzz-terminal/src/lib.rs

@@ -158,6 +158,7 @@ impl Terminal {
     }
 
     /// Whether the tail is at its cap and the reader must stop reading.
+    /// Not yet consumed in production -- see [`reader::Feeder::tail_full`].
     pub fn tail_full(&self) -> bool {
         self.feeder.tail_full()
     }

desktop/src-tauri/crates/buzz-terminal/src/reader.rs

@@ -111,6 +111,15 @@ impl Feeder {
     /// latch is a state the fence owns and could fail to clear, which is
     /// exactly how a paused reader strands a child mid-teardown; a reader that
     /// simply stops asking resumes by default.
+    ///
+    /// **Not yet consumed in production.** The runtime reader pumps
+    /// [`Feeder::drain`] to completion after every read, so the tail cannot
+    /// currently grow to the cap and nothing needs to ask. This signal exists
+    /// for the reader that stops pumping -- it is the queue bound, and the
+    /// pump loop is the only reason the queue bound is not load-bearing
+    /// today. Stated rather than left to be inferred from an empty
+    /// call-graph: an unused signal that looks wired is worse than one that
+    /// says it isn't.
     pub fn tail_full(&self) -> bool {
         self.pending_bytes() >= TAIL_CAP
     }