namespace-syntax-introduce: fix bad phase shift

A `namespace-syntax-introduce` at a phase other than 0 could shift the
phase of introduced context by double the correct shift.

Relevant to #2099

pkgs/racket-test-core/tests/racket/module.rktl

@@ -2654,6 +2654,22 @@ case of module-leve bindings; it doesn't cover local bindings.
 
 (test 'ok dynamic-require ''discards-module-begin-macro-definition 'result)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Make sure a compile-time `eval` (as opposed to `eval-syntax`)
+;; doesn't add the wrong phase's bindings via
+;; `namespace-syntax-introduce`. The wrong phase's bindings in this
+;; example could make `#%app` ambiguous.
+
+(module uses-eval-at-compile-time racket/base
+  (require (for-syntax racket/base)
+           (for-meta 2 racket/base))
+  
+  (define-syntax (ct-eval stx)
+    (syntax-case stx ()
+      [(_ e) #`'#,(eval #'e)]))
+  
+  (ct-eval (+ 1 2)))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/expander/namespace/api.rkt

@@ -62,7 +62,7 @@
   (define other-namespace-scopes (for/list ([sc (in-set
                                                  ;; `all-scopes-stx` corresponds to the initial import
                                                  (syntax-scope-set (root-expand-context-all-scopes-stx root-ctx)
-                                                                   (namespace-phase ns)))]
+                                                                   0))]
                                             #:unless (equal? sc post-scope))
                                    sc))
   (define (add-ns-scopes s)

racket/src/racket/src/startup.inc

@@ -44047,7 +44047,7 @@ static const char *startup_source =
 "(let-values(((ht_148)"
 "(syntax-scope-set"
 "(root-expand-context-all-scopes-stx root-ctx_5)"
-"(namespace-phase ns_58))))"
+" 0)))"
 "(begin"
 "(if(variable-reference-from-unsafe?(#%variable-reference))"
 "(void)"