apl: quick-wins bundle — decimals + ⎕← + strings (+10 tests, 460/460)

Three small unblockers in one iteration:
- tokenizer: read-digits! now consumes optional ".digits" suffix,
  so 3.7 and ¯2.5 are single number tokens.
- tokenizer: ⎕ followed by ← emits a single :name "⎕←" token
  (instead of splitting on the assign glyph).  Parser registers
  ⎕← in apl-quad-fn-names; apl-monadic-fn maps to apl-quad-print.
- eval-ast: :str AST nodes evaluate to char arrays.  Single-char
  strings become rank-0 scalars; multi-char become rank-1 vectors
  of single-char strings.

lib/apl/parser.sx

@@ -82,7 +82,7 @@
     "⍎"
     "⍕"))
 
-(define apl-quad-fn-names (list "⎕FMT"))
+(define apl-quad-fn-names (list "⎕FMT" "⎕←"))
 
 (define
   apl-parse-op-glyph?