apl: train/fork notation (f g h) and (g h) (+6 tests, 496/496)

Parser: when a parenthesised subexpression contains only function
segments (>= 2), collect-segments-loop now emits a :train AST node
instead of treating it as a value-producing expression.

Resolver: apl-resolve-{monadic,dyadic} handle :train.
- monadic 2-train (atop):  (g h)⍵ = g (h ⍵)
- monadic 3-train (fork):  (f g h)⍵ = (f ⍵) g (h ⍵)
- dyadic 2-train:          ⍺(g h)⍵ = g (⍺ h ⍵)
- dyadic 3-train:          ⍺(f g h)⍵ = (⍺ f ⍵) g (⍺ h ⍵)

apl-run "(+/÷≢) 1 2 3 4 5"  → 3   (mean)
apl-run "(- ⌊) 5"           → -5  (atop)
apl-run "2 (+ × -) 5"       → -21 (dyadic fork)
apl-run "(⌈/-⌊/) 3 1 4 …"   → 8   (range)

lib/apl/tests/pipeline.sx

@@ -282,3 +282,33 @@
   "multi-axis: select all rows of column 3"
   (mkrv (apl-run "M ← (4 3) ⍴ 1 2 3 4 5 6 7 8 9 10 11 12 ⋄ M[;3]"))
   (list 3 6 9 12))
+
+(apl-test
+  "train: mean = (+/÷≢) on 1..5"
+  (mkrv (apl-run "(+/÷≢) 1 2 3 4 5"))
+  (list 3))
+
+(apl-test
+  "train: mean of 2 4 6 8 10"
+  (mkrv (apl-run "(+/÷≢) 2 4 6 8 10"))
+  (list 6))
+
+(apl-test
+  "train 2-atop: (- ⌊) 5 → -5"
+  (mkrv (apl-run "(- ⌊) 5"))
+  (list -5))
+
+(apl-test
+  "train 3-fork dyadic: 2(+×-)5 → -21"
+  (mkrv (apl-run "2 (+ × -) 5"))
+  (list -21))
+
+(apl-test
+  "train: range = (⌈/-⌊/) on vector"
+  (mkrv (apl-run "(⌈/-⌊/) 3 1 4 1 5 9 2 6"))
+  (list 8))
+
+(apl-test
+  "train: mean of ⍳10 has shape ()"
+  (mksh (apl-run "(+/÷≢) ⍳10"))
+  (list))