Merge final terminal backpressure documentation

Co-authored-by: npub1jh9wn95s0472h86ahapupaf7m6kx4v9sx2n0atj2hltcfer8k06s5n3pyf <95cae996907d7cab9f5dbf43c0f53edeac6ab0b032a6feae4abfd784e467b3f5@buzz.block.builderlab.xyz>

Signed-off-by: npub1jh9wn95s0472h86ahapupaf7m6kx4v9sx2n0atj2hltcfer8k06s5n3pyf <95cae996907d7cab9f5dbf43c0f53edeac6ab0b032a6feae4abfd784e467b3f5@buzz.block.builderlab.xyz>

* commit '232f56b67219613ddb1fd6628a73ae31bd1156a5':
  docs(terminal): say why tail_full cannot fire, not just that nothing calls it

Signed-off-by: npub1jh9wn95s0472h86ahapupaf7m6kx4v9sx2n0atj2hltcfer8k06s5n3pyf <95cae996907d7cab9f5dbf43c0f53edeac6ab0b032a6feae4abfd784e467b3f5@buzz.block.builderlab.xyz>

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

@@ -112,14 +112,19 @@ impl Feeder {
     /// 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.
+    /// **No production consumer today, and not an oversight.** The runtime
+    /// reader pumps [`Feeder::drain`] to completion after every read
+    /// (`terminal_runtime.rs`), so the tail is empty between iterations and
+    /// this can never go true -- measured 0 bytes high-water against 1 MiB of
+    /// pure RIS, the densest atom there is. It exists for a future reader
+    /// that defers pumping, and such a reader **must** consult it: without
+    /// the pump loop the same stream reaches [`TAIL_CAP`] in 257 reads of
+    /// 16 KiB.
+    ///
+    /// The numbers are here rather than "nothing calls this" because the
+    /// signal and the loop are one fact from two sides. Delete the loop and
+    /// this predicate stops being unreachable in the same instant it starts
+    /// being needed.
     pub fn tail_full(&self) -> bool {
         self.pending_bytes() >= TAIL_CAP
     }