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/transpile.sx

@@ -40,6 +40,7 @@
       ((= g "⍋") apl-grade-up)
       ((= g "⍒") apl-grade-down)
       ((= g "⎕FMT") apl-quad-fmt)
+      ((= g "⎕←") apl-quad-print)
       (else (error "no monadic fn for glyph")))))
 
 (define
@@ -97,6 +98,15 @@
       ((tag (first node)))
       (cond
         ((= tag :num) (apl-scalar (nth node 1)))
+        ((= tag :str)
+          (let
+            ((s (nth node 1)))
+            (if
+              (= (len s) 1)
+              (apl-scalar s)
+              (make-array
+                (list (len s))
+                (map (fn (i) (slice s i (+ i 1))) (range 0 (len s)))))))
         ((= tag :vec)
           (let
             ((items (rest node)))