Numbered headings

The manual numbers its parts, sections and subsections, 1, 1.1, 1.1.1, in
its table of contents too. The titles that numbered themselves no longer
do: Part III is "The standard library", 3, and the appendices are 4.1 to
4.4 rather than A to D. The std/random section after the appendices is
gone; Part III has it.

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Co-Authored-By: eca-agent <git@eca.dev>

manual.sz

@@ -1,13 +1,10 @@
 (meta (title "The Bjolang manual")
       (date "2026-09-29"))
 
+@numbered-headings
 @toc[#:depth 3]
 
 @include["_01.Language-basics.sz"]
 @include["_02_.Writing-programs.sz"]
 @include["_03.Standard-library.sz"]
 @include["_04.Appendices.sz"]
-
-@subsection{std/random}
-
-@defmodule[(std random)]