Bring the formal definition in line with this session's language work:
- grammar.ebnf: numbers are a real tower (exact integer / Ratio / double); the M
suffix reads a real BigDecimal, N an exact integer (drop the stale Janet note).
- 02-reader S5: M is a real java.math.BigDecimal with scale-insensitive equality.
- 03-special-forms: document the read -> macroexpand -> analyze order (macros
expand before special-form dispatch); special-form heads are not shadowable but
macros are and value-position locals may be named like a special; set! on a var
sets the innermost binding (else root); letfn is a primitive with letrec*
semantics.