Phase 7e: polymorphic print(x) + str(x) (retire print_int/print_str)

One `print` instead of two C-style names: `print(x)` writes an int OR a string
followed by a newline, dispatching on the operand type (int -> %d, string ->
%s). `print(int)` emits byte-identically to the old print_int, so every existing
call and every smoke-test output is unchanged.

print_str was only ever the raw IR-to-stdout dump in ir_flush (no newline), which
is not "printing a line" — so it now uses file_write to a new file_stdout()
stream, keeping the emitted IR byte-for-byte identical. That frees `print` to
have consistent always-newline semantics.

Two reseeds: (A) add print + str + file_stdout keeping the intrinsics; (B)
migrate the 61 print_int calls to print, ir_flush to file_write(file_stdout()),
and delete print_int/print_str (+ the now-dead @.fmt_str). str(x) (the
interpolation converter from 7d) is now also a documented standalone builtin.

Vocabulary: print/str/file_stdout in, print_int/print_str out (ludic_syntax.h,
grammar, LudicTokens.kt). LANGUAGE.md updated. Reseeded (22551 lines); C-free
fixpoint holds; goldens identical; 18/18; vocab + doc-fences clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/tests/structs.ludic

@@ -3,18 +3,18 @@ program T {
   fn bump(p: P) -> void { p.x = p.x + 100 }
   entry {
     let a = new P
-    print_int(a.y)            # 7 default
+    print(a.y)            # 7 default
     a.x = 5
     let b = a
     b.x = 9
-    print_int(a.x)            # 9 (reference)
+    print(a.x)            # 9 (reference)
     bump(a)
-    print_int(a.x)            # 109
+    print(a.x)            # 109
     let c = new P
     c.x = 2
     a.next = c
-    print_int(a.next.x)      # 2 chained
+    print(a.next.x)      # 2 chained
     a.next.x = 42
-    print_int(c.x)           # 42
+    print(c.x)           # 42
   }
 }