apl: ⍵-rebind + primes.apl runs as-written (+4 tests)

Two changes wire the original primes idiom through:

1. Parser :glyph branch detects ⍵← / ⍺← and emits :assign-expr
   (was only :name-token before).
2. Eval-ast :name lookup checks env["⍵"]/env["⍺"] before falling
   back to env["omega"]/env["alpha"].  Inline ⍵-rebind binds
   under the glyph key directly.

apl-run "primes ← {(2=+⌿0=⍵∘.|⍵)/⍵←⍳⍵} ⋄ primes 50"
→ 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

primes.apl now runs as-written via apl-run-file + " ⋄ primes 30".

lib/apl/transpile.sx

@@ -122,8 +122,14 @@
           (let
             ((nm (nth node 1)))
             (cond
-              ((= nm "⍺") (get env "alpha"))
-              ((= nm "⍵") (get env "omega"))
+              ((= nm "⍺")
+                (let
+                  ((v (get env "⍺")))
+                  (if (= v nil) (get env "alpha") v)))
+              ((= nm "⍵")
+                (let
+                  ((v (get env "⍵")))
+                  (if (= v nil) (get env "omega") v)))
               ((= nm "⎕IO") (apl-quad-io))
               ((= nm "⎕ML") (apl-quad-ml))
               ((= nm "⎕FR") (apl-quad-fr))