docs: M5d-2 hold returns Busy, not Result<(), ()>

clippy result_unit_err is denied by the gate. Context::hold uses a Busy
unit struct; the given tests still only expect Ok.

Co-Authored-By: Grok 4.6 <noreply@x.ai>

docs/plans/M5d2/06-loopd-scheduler-heartbeat.md

@@ -40,9 +40,9 @@ Tests call `tick(ctx, now)` with injected time. No sleeps (T27).
 
 ## Facts you need
 
-- **`Context::hold`.** Same insert as `handle`: if the id is already in `busy`, `Err(())`;
+- **`Context::hold`.** Same insert as `handle`: if the id is already in `busy`, `Err(Busy)`;
   else insert and return `Held`. `Held` must be `pub` so the given test can keep it. `Drop`
-  already removes the id.
+  already removes the id. Do not return `Result<_, ()>`: clippy `result_unit_err` is denied.
 - **Due.** `every_s == 0` → no heartbeat work. Else `now.unix_millis().saturating_sub(last.unix_millis()) >= every_s.saturating_mul(1000)`
   where `last` is `last_heartbeat.unwrap_or(started)`. `07:00` to `07:01` with `every_s = 60`
   is due. `07:01` then `07:01:30` is not.
@@ -101,17 +101,24 @@ Tests call `tick(ctx, now)` with injected time. No sleeps (T27).
 
 ### `crates/loopd/src/channel.rs`
 
-**C1.** `struct Held` becomes `pub struct Held`.
+**C1.** `struct Held` becomes `pub struct Held`. Immediately after that struct (before
+`impl Drop for Held`), add:
+
+```rust
+/// The session is already in the busy set.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Busy;
+```
 
 **C2.** Inside `impl Context`, after `new`, add:
 
 ```rust
-    /// Mark `id` busy. `Err(())` if it already is. Tests and the scheduler use this so a skip
+    /// Mark `id` busy. `Err(Busy)` if it already is. Tests and the scheduler use this so a skip
     /// is not a sleep (T27).
-    pub fn hold(&self, id: SessionId) -> Result<Held<'_>, ()> {
+    pub fn hold(&self, id: SessionId) -> Result<Held<'_>, Busy> {
         let mut busy = self.busy.lock().unwrap_or_else(|p| p.into_inner());
         if busy.contains(&id) {
-            return Err(());
+            return Err(Busy);
         }
         busy.insert(id.clone());
         drop(busy);
@@ -206,7 +213,8 @@ pass so it runs once.
 
 - The branch check fails; a "before" block is not found exactly once; step 2 differs; `cargo
   check` warns; a test fails; the gate count is not 1551; you sleep in a test; you post to
-  Mattermost; you send `slots.main`; you set a speaker; you `chmod 000`; `Held` stays private.
+  Mattermost; you send `slots.main`; you set a speaker; you `chmod 000`; `Held` stays private;
+  `hold` returns `Result<_, ()>`.
 
 ## What the given tests pin