spec: string ports (open-input-string/open-output-string/read-char/etc)

Phase 14: port type + eof-object. Input ports track _pos cursor; output ports
accumulate _buffer. All 15 port primitives in spec/primitives.sx (stdlib.ports
module), platform.py (JS), and 39/39 tests in spec/tests/test-ports.sx.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -856,4 +856,96 @@
   :returns "number"
   :doc "Number of bits needed to represent integer a (excluding sign).")
 
+(define-module :stdlib.ports)
+
+(define-primitive
+  "eof-object"
+  :params ()
+  :returns "eof-object"
+  :doc "The EOF sentinel value.")
+
+(define-primitive
+  "eof-object?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is the EOF sentinel.")
+
+(define-primitive
+  "open-input-string"
+  :params ((s :as string))
+  :returns "input-port"
+  :doc "Open a string as an input port.")
+
+(define-primitive
+  "open-output-string"
+  :params ()
+  :returns "output-port"
+  :doc "Open a fresh output string port.")
+
+(define-primitive
+  "get-output-string"
+  :params ((p :as output-port))
+  :returns "string"
+  :doc "Flush output port contents to a string.")
+
+(define-primitive
+  "port?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is any port.")
+
+(define-primitive
+  "input-port?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is an input port.")
+
+(define-primitive
+  "output-port?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is an output port.")
+
+(define-primitive
+  "close-port"
+  :params ((p :as port))
+  :returns "nil"
+  :doc "Close a port.")
+
+(define-primitive
+  "read-char"
+  :params (&rest (p :as input-port))
+  :returns "any"
+  :doc "Read next char from port; returns eof-object at end.")
+
+(define-primitive
+  "peek-char"
+  :params (&rest (p :as input-port))
+  :returns "any"
+  :doc "Peek next char without consuming; returns eof-object at end.")
+
+(define-primitive
+  "read-line"
+  :params (&rest (p :as input-port))
+  :returns "any"
+  :doc "Read a line from port; returns eof-object at end.")
+
+(define-primitive
+  "write-char"
+  :params ((c :as char) &rest (p :as output-port))
+  :returns "nil"
+  :doc "Write a char to output port.")
+
+(define-primitive
+  "write-string"
+  :params ((s :as string) &rest (p :as output-port))
+  :returns "nil"
+  :doc "Write a string to output port.")
+
+(define-primitive
+  "char-ready?"
+  :params (&rest (p :as input-port))
+  :returns "boolean"
+  :doc "True if a char is immediately available on the port.")
+
 (define-module :stdlib.hash-table)