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/tests/pipeline.sx

@@ -178,3 +178,32 @@
   "apl-run \"(⍳5)[3] × 7\" → 21"
   (mkrv (apl-run "(⍳5)[3] × 7"))
   (list 21))
+
+(apl-test "decimal: 3.7 → 3.7" (mkrv (apl-run "3.7")) (list 3.7))
+
+(apl-test "decimal: ¯2.5 → -2.5" (mkrv (apl-run "¯2.5")) (list -2.5))
+
+(apl-test "decimal: 1.5 + 2.5 → 4" (mkrv (apl-run "1.5 + 2.5")) (list 4))
+
+(apl-test "decimal: ⌊3.7 → 3" (mkrv (apl-run "⌊ 3.7")) (list 3))
+
+(apl-test "decimal: ⌈3.7 → 4" (mkrv (apl-run "⌈ 3.7")) (list 4))
+
+(apl-test
+  "⎕← scalar passthrough"
+  (mkrv (apl-run "⎕← 42"))
+  (list 42))
+
+(apl-test
+  "⎕← vector passthrough"
+  (mkrv (apl-run "⎕← 1 2 3"))
+  (list 1 2 3))
+
+(apl-test
+  "string: 'abc' → 3-char vector"
+  (mkrv (apl-run "'abc'"))
+  (list "a" "b" "c"))
+
+(apl-test "string: 'a' is rank-0 scalar" (mksh (apl-run "'a'")) (list))
+
+(apl-test "string: 'hello' shape (5)" (mksh (apl-run "'hello'")) (list 5))