docs: document array[NamedSubrange] index type

grammar.ebnf: the ARRAY [IDENT] OF T form is an ordinal-indexed array;
clarify it admits both an enumeration type and a named integer subrange
as the index, with a note on how each folds (enum -> array[0..N-1],
subrange -> array[lo..hi]).

language-rationale.adoc: add the array[TSubrange] of T row to the
supported array-index table, noting the subrange supplies the index
range while its values remain ordinary unchecked integers.

docs/grammar.ebnf

@@ -738,9 +738,18 @@ QualIdent
 ArrayType
     = ARRAY LBRACKET ArrayBound DOTDOT ArrayBound
             { COMMA ArrayBound DOTDOT ArrayBound } RBRACKET OF TypeName
-    | ARRAY LBRACKET IDENT RBRACKET OF TypeName   (* ordinal/enum-indexed *)
+    | ARRAY LBRACKET IDENT RBRACKET OF TypeName   (* ordinal-indexed *)
     ;
 
+(* The IDENT form names an ORDINAL index type.  Two ordinal kinds are admitted:
+ *   - an enumeration type (array[TColor] of T folds to array[0..N-1] of T,
+ *     where N is the member count), and
+ *   - a NAMED integer subrange (type TIdx = lo..hi;  array[TIdx] of T folds to
+ *     array[lo..hi] of T).  The subrange supplies only the index range; the
+ *     values it yields remain ordinary unchecked integers of the subrange's
+ *     underlying storage type.
+ * See language-rationale.adoc "Array index types". *)
+
 ArrayBound
     = ConstArithExpr
     ;

docs/language-rationale.adoc

@@ -1154,6 +1154,9 @@ Blaise supports fixed-size, stack-allocated arrays declared as
 | `array[TEnum] of T` — enum-type index
 | Supported
 
+| `array[TSubrange] of T` — named integer-subrange index
+| Supported. The subrange (`type TIdx = lo..hi;`) supplies the index range, so `array[TIdx] of T` folds to `array[lo..hi] of T`; the subrange's values remain ordinary unchecked integers of its underlying storage type.
+
 | `array[L..H, M..N] of T` — multi-dimensional
 | Supported