apl: bracket indexing A[I] → (I⌷A) (+7 tests, 415/415)

Parser: maybe-bracket helper wraps any value followed by [expr]
into (:dyad (:fn-glyph ⌷) idx val).  Wired into :name and :lparen
branches of collect-segments-loop.

apl-run "(10 20 30)[2]" → 20
apl-run "A ← 100 200 300 ⋄ A[2]" → 200
apl-run "(⍳5)[3] × 7" → 21

Multi-axis A[I;J] deferred — needs semicolon-split parsing.

lib/apl/tests/pipeline.sx

@@ -143,3 +143,38 @@
   "1 2 3 4 5")
 
 (apl-test "apl-run \"⎕IO + 4\" → 5" (mkrv (apl-run "⎕IO + 4")) (list 5))
+
+(apl-test
+  "apl-run \"(10 20 30 40 50)[3]\" → 30"
+  (mkrv (apl-run "(10 20 30 40 50)[3]"))
+  (list 30))
+
+(apl-test
+  "apl-run \"(⍳10)[5]\" → 5"
+  (mkrv (apl-run "(⍳10)[5]"))
+  (list 5))
+
+(apl-test
+  "apl-run \"A ← 100 200 300 ⋄ A[2]\" → 200"
+  (mkrv (apl-run "A ← 100 200 300 ⋄ A[2]"))
+  (list 200))
+
+(apl-test
+  "apl-run \"V ← ⍳10 ⋄ V[3]\" → 3"
+  (mkrv (apl-run "V ← ⍳10 ⋄ V[3]"))
+  (list 3))
+
+(apl-test
+  "apl-run \"(10 20 30)[1]\" → 10 (1-indexed)"
+  (mkrv (apl-run "(10 20 30)[1]"))
+  (list 10))
+
+(apl-test
+  "apl-run \"V ← 10 20 30 40 50 ⋄ V[3] + 1\" → 31"
+  (mkrv (apl-run "V ← 10 20 30 40 50 ⋄ V[3] + 1"))
+  (list 31))
+
+(apl-test
+  "apl-run \"(⍳5)[3] × 7\" → 21"
+  (mkrv (apl-run "(⍳5)[3] × 7"))
+  (list 21))