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/parser.sx

@@ -344,10 +344,22 @@
             ((= tt :glyph)
               (cond
                 ((or (= tv "⍺") (= tv "⍵"))
-                  (collect-segments-loop
-                    tokens
-                    (+ i 1)
-                    (append acc {:kind "val" :node (list :name tv)})))
+                  (if
+                    (and
+                      (< (+ i 1) (len tokens))
+                      (= (tok-type (nth tokens (+ i 1))) :assign))
+                    (let
+                      ((rhs-tokens (slice tokens (+ i 2) (len tokens))))
+                      (let
+                        ((rhs-expr (parse-apl-expr rhs-tokens)))
+                        (collect-segments-loop
+                          tokens
+                          (len tokens)
+                          (append acc {:kind "val" :node (list :assign-expr tv rhs-expr)}))))
+                    (collect-segments-loop
+                      tokens
+                      (+ i 1)
+                      (append acc {:kind "val" :node (list :name tv)}))))
                 ((= tv "∇")
                   (collect-segments-loop
                     tokens