docs(nix): narrow let binding rules

docs/nix.md

@@ -131,10 +131,10 @@ Use:
 
 ## `let` Bindings
 
-### Name the role, not the type
+### Use role-specific binding names
 
-Use a `let` binding when the name explains the value's role.
-Do not introduce a vague name only to shorten an expression.
+A `let` binding name must explain the value's role in the expression.
+Avoid names that describe only the value's type.
 
 Avoid:
 
@@ -154,10 +154,9 @@ in
 lib.genAttrs hostNames mkHost
 ```
 
-### Share important repeated values
+### Bind shared expressions once
 
-Use a `let` binding when multiple expressions share one important value.
-Do not repeat an expression when repetition makes the relationship less clear.
+Use a `let` binding when sibling expressions depend on the same derived value.
 
 Avoid:
 
@@ -180,10 +179,9 @@ in
 }
 ```
 
-### Keep bindings near their use
+### Keep bindings in the smallest scope
 
-Place a `let` binding in the smallest expression that needs it.
-Do not lift a value to a larger scope only for convenience.
+Place a `let` binding inside the expression that consumes it.
 
 Avoid:
 
@@ -192,7 +190,10 @@ let
   hostNames = lib.attrNames hosts;
 in
 {
-  hostCount = lib.length hostNames;
+  hostSummary = {
+    inherit hostNames;
+    count = lib.length hostNames;
+  };
   nixosConfigurations = flakeAssembly.mkHosts (self + "/hosts");
 }
 ```
@@ -201,7 +202,14 @@ Use:
 
 ```nix
 {
-  hostCount = lib.length (lib.attrNames hosts);
+  hostSummary =
+    let
+      hostNames = lib.attrNames hosts;
+    in
+    {
+      inherit hostNames;
+      count = lib.length hostNames;
+    };
   nixosConfigurations = flakeAssembly.mkHosts (self + "/hosts");
 }
 ```