jit: ljust/rjust/center truncate when width < content (#772)

Found by the JIT differential fuzzer (random nested softcode, JIT vs
interpreter).

fun_ljust/fun_rjust/fun_center call centerjustcombo(..., bTrunc=true), which
passes 1 to co_ljust/co_rjust/co_center → a too-wide string is truncated to
the width.  The rv64 blob wrappers co_ljust_wrap/co_rjust_wrap/co_center_wrap
passed 0 (no truncation), so the JIT/DBT path returned the full string where
the interpreter truncates:

  ljust(abcdef,3)   interp: abc   JIT (blob): abcdef
  rjust(abcdef,3)   interp: abc   JIT (blob): abcdef
  center(abcdef,3)  interp: abc   JIT (blob): abcdef

(The padding case width > content already agreed, which is why the earlier
flat audit missed this.)

Change the final bTrunc arg from 0 to 1 in all three wrappers and rebuild
the rv64 softlib blob.  This also matches the interpreter's empty-string
returns for width 0 and non-integer width: satoi→0 then truncate-to-0 → "".
There is no host-side fold for these (hir_lower has no LJUST/RJUST/CENTER
constant-fold), so the wrapper is the whole runtime path.

Tests: add TC006 to ljust_fn/rjust_fn/center_fn asserting truncation
(abcdef,3 → abc) plus width-0 and non-integer → "".  Because these
functions are not constant-folded, the JIT-compiled @if condition routes
them through the tier2 blob wrappers, so the tests are genuinely
fail-closed against this bug — verified by building the unfixed blob and
confirming all three TC006 cases fail, then pass after the fix.

Smoke (configured --enable-jit, runtime JIT confirmed active, tier2=872):
1086 / 0 failed / 0 crashes.

Closes #772.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mux/rv64/src/softlib.c

@@ -865,9 +865,12 @@ char *co_ljust_wrap(char *out, const char **fargs, int nfargs) {
         fill = (const unsigned char *)fargs[2];
         fill_len = rv64_slen(fargs[2]);
     }
+    /* bTrunc=1: fun_ljust truncates when width < content (centerjustcombo
+     * passes bTrunc=true).  satoi→0 for width 0 / non-integer then truncates
+     * to "", matching the interpreter's empty returns.  See #772. */
     size_t n = co_ljust((unsigned char *)out,
                         (const unsigned char *)fargs[0], len,
-                        (size_t)width, fill, fill_len, 0);
+                        (size_t)width, fill, fill_len, 1);
     out[n] = '\0';
     return out;
 }
@@ -884,9 +887,10 @@ char *co_rjust_wrap(char *out, const char **fargs, int nfargs) {
         fill = (const unsigned char *)fargs[2];
         fill_len = rv64_slen(fargs[2]);
     }
+    /* bTrunc=1: see co_ljust_wrap above (#772). */
     size_t n = co_rjust((unsigned char *)out,
                         (const unsigned char *)fargs[0], len,
-                        (size_t)width, fill, fill_len, 0);
+                        (size_t)width, fill, fill_len, 1);
     out[n] = '\0';
     return out;
 }
@@ -903,9 +907,10 @@ char *co_center_wrap(char *out, const char **fargs, int nfargs) {
         fill = (const unsigned char *)fargs[2];
         fill_len = rv64_slen(fargs[2]);
     }
+    /* bTrunc=1: see co_ljust_wrap above (#772). */
     size_t n = co_center((unsigned char *)out,
                          (const unsigned char *)fargs[0], len,
-                         (size_t)width, fill, fill_len, 0);
+                         (size_t)width, fill, fill_len, 1);
     out[n] = '\0';
     return out;
 }

testcases/center_fn.mux

@@ -94,11 +94,30 @@
         strmatch(center(X,7,-+), -+-X-+-)
       )=
   {
-    @log smoke=TC005: Vary string and width with '-+' fill. Succeeded.;
+    @log smoke=TC005: Vary string and width with '-+' fill. Succeeded.
+  },
+  {
+    @log smoke=TC005: Vary string and width with '-+' fill. Failed (c2=[center(X,5,-+)] c3=[center(X,7,-+)]).
+  }
+-
+#
+# Test Case #6 - Truncation when width < content (#772).  fun_center passes
+# bTrunc=true, so a too-wide string is truncated; width 0 and non-integer
+# width collapse to empty.  Previously untested.  On the JIT/DBT blob
+# backend this also guards co_center_wrap.
+#
+&tr.tc006 test_center_fn=
+  @if cand(
+        strmatch(center(abcdef,3), abc),
+        eq(strlen(center(abcdef,0)), 0),
+        eq(strlen(center(abcdef,xyz)), 0)
+      )=
+  {
+    @log smoke=TC006: center truncation. Succeeded.;
     @trig me/tr.done
   },
   {
-    @log smoke=TC005: Vary string and width with '-+' fill. Failed (c2=[center(X,5,-+)] c3=[center(X,7,-+)]).;
+    @log smoke=TC006: center truncation. Failed (t=[center(abcdef,3)] w0=[center(abcdef,0)] wx=[center(abcdef,xyz)]).;
     @trig me/tr.done
   }
 -

testcases/ljust_fn.mux

@@ -90,11 +90,30 @@
         eq(strlen(ljust(123,11,-+)), 11)
       )=
   {
-    @log smoke=TC005: Vary string and width with '-+' fill. Succeeded.;
+    @log smoke=TC005: Vary string and width with '-+' fill. Succeeded.
+  },
+  {
+    @log smoke=TC005: Vary string and width with '-+' fill. Failed.
+  }
+-
+#
+# Test Case #6 - Truncation when width < content (#772).  fun_ljust passes
+# bTrunc=true, so a too-wide string is truncated; width 0 and non-integer
+# width collapse to empty.  Previously untested.  On the JIT/DBT blob
+# backend this also guards co_ljust_wrap, which was the divergent path.
+#
+&tr.tc006 test_ljust_fn=
+  @if cand(
+        strmatch(ljust(abcdef,3), abc),
+        eq(strlen(ljust(abcdef,0)), 0),
+        eq(strlen(ljust(abcdef,xyz)), 0)
+      )=
+  {
+    @log smoke=TC006: ljust truncation. Succeeded.;
     @trig me/tr.done
   },
   {
-    @log smoke=TC005: Vary string and width with '-+' fill. Failed.;
+    @log smoke=TC006: ljust truncation. Failed (t=[ljust(abcdef,3)] w0=[ljust(abcdef,0)] wx=[ljust(abcdef,xyz)]).;
     @trig me/tr.done
   }
 -

testcases/rjust_fn.mux

@@ -90,11 +90,30 @@
         eq(strlen(rjust(123,11,-+)), 11)
       )=
   {
-    @log smoke=TC005: Vary string and width with '-+' fill. Succeeded.;
+    @log smoke=TC005: Vary string and width with '-+' fill. Succeeded.
+  },
+  {
+    @log smoke=TC005: Vary string and width with '-+' fill. Failed.
+  }
+-
+#
+# Test Case #6 - Truncation when width < content (#772).  fun_rjust passes
+# bTrunc=true, so a too-wide string is truncated (to the leading chars);
+# width 0 and non-integer width collapse to empty.  Previously untested.
+# On the JIT/DBT blob backend this also guards co_rjust_wrap.
+#
+&tr.tc006 test_rjust_fn=
+  @if cand(
+        strmatch(rjust(abcdef,3), abc),
+        eq(strlen(rjust(abcdef,0)), 0),
+        eq(strlen(rjust(abcdef,xyz)), 0)
+      )=
+  {
+    @log smoke=TC006: rjust truncation. Succeeded.;
     @trig me/tr.done
   },
   {
-    @log smoke=TC005: Vary string and width with '-+' fill. Failed.;
+    @log smoke=TC006: rjust truncation. Failed (t=[rjust(abcdef,3)] w0=[rjust(abcdef,0)] wx=[rjust(abcdef,xyz)]).;
     @trig me/tr.done
   }
 -