Make p paste as children and P as siblings below

After trying part 8c, the user found p and P doing the same thing on the
canvas: both pasted a sibling below the node (P only differed in the
outline's order). Their pick: p pastes under the node, as its last
children, the way a adds a child; P pastes the next siblings, below it,
where o adds one. ]p went away, since p does what it did.

Inside a title p and P still paste after and before the text cursor, as
in vim: they're their own commands (paste-after, paste-before), and the
node ones are paste-as-child and paste-as-sibling.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/action.rs

@@ -145,7 +145,7 @@ pub enum AppAction {
         nodes: Vec<NodeId>,
         branches: bool,
     },
-    /// Paste what a register holds (`p`, `P`, `]p`): its nodes, or its text
+    /// Paste what a register holds (`p`, `P`): its nodes, or its text
     /// made into nodes, `times` copies.
     Paste {
         register: Option<Register>,
@@ -256,15 +256,14 @@ pub enum AppAction {
     Say(Message),
 }
 
-/// Where `p` and its kin paste, from the cursor's node.
+/// Where `p` and `P` paste, from the cursor's node (the user's pick after
+/// trying part 8c: `p` pastes under the node, as `a` adds).
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum PasteAt {
-    /// Its next siblings (`p`).
-    After,
-    /// Its siblings before it (`P`).
-    Before,
-    /// Its last children (`]p`).
+    /// Its last children (`p`).
     Child,
+    /// Its next siblings, below it, where `o` adds one (`P`).
+    Sibling,
 }
 
 /// What `:w` and its kin save.