sampling: guard against non-finite logits (was silent token-0 spew)

On the default serve path (TEMP>0, 0<NUCLEUS<1) a single NaN or +Inf in
the logits — a bad streamed expert tile, or an fp overflow in the matmul
at a low-RAM eviction boundary — poisoned softmax: g_pbuf became all-NaN,
dist_sample never satisfied cum>=u, and the fallback returned token 0. The
engine then emitted an unbroken run of token 0 with NO error. The greedy
path was equally blind: argmax_v started bv=lo[0] and `lo[i]>NaN` is always
false, so a NaN at index 0 pinned the argmax to 0.

- argmax_v: skip NaN (x==x) and seed from -inf, so it returns the max
  finite/+Inf entry instead of being NaN-pinned to 0. Covers greedy decode
  and the speculative-verify argmax path.
- dist_build: after the softmax sum, if s is non-finite or <=0, collapse
  g_pbuf to a one-hot over the finite argmax and warn once, instead of
  dividing every entry into NaN. Covers the nucleus and verify paths.

Both are O(1)/free on the happy path (one branch after the existing loop;
one extra comparison inside the existing argmax loop). Degrade + diagnose,
never silently corrupt.

test_logit_nan (wired into TEST_BINS): asserts argmax_v skips NaN/picks
+Inf, dist_build yields a finite normalized one-hot on the max finite
logit, dist_sample emits that token (not 0), and clean logits still give a
valid distribution. Fails on stock dev, passes with this change.

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

c/tests/test_logit_nan.c

@@ -0,0 +1,55 @@
+/* Regression for non-finite-logit poisoning of sampling.
+ *
+ * A single NaN or +Inf in the logits (a bad streamed expert tile, or an fp
+ * overflow in the matmul at a low-RAM eviction boundary) used to make softmax
+ * produce an all-NaN g_pbuf; dist_sample then never satisfied cum>=u and fell
+ * through to return token 0 — so the engine silently emitted an unbroken run of
+ * token 0 with no error, on the DEFAULT serve path (TEMP>0, 0<NUCLEUS<1).
+ *
+ * Fix under test: argmax_v() skips NaN (picks the max finite/+Inf entry instead
+ * of being pinned to index 0), and dist_build() detects a non-finite softmax sum
+ * and collapses to a one-hot over the finite argmax (warning once) rather than
+ * dividing every entry into NaN. Degrade + diagnose, never silently corrupt.
+ *
+ * No model file needed: exercises argmax_v / dist_build / dist_sample directly. */
+#include <assert.h>
+#include <math.h>
+#define main coli_glm_main_unused
+#include "../glm.c"
+#undef main
+
+static int approx1(double x){ return x > 0.999 && x < 1.001; }
+
+int main(void){
+    /* --- argmax_v must skip NaN (greedy decode + speculative-verify paths) --- */
+    { float lo[8]={NAN,1.f,5.f,2.f,NAN,-3.f,4.f,0.f};
+      assert(argmax_v(lo,8)==2 && "pick max finite (idx2=5.0), not NaN-pinned idx0"); }
+    { float lo[8]={3.f,INFINITY,1.f,2.f,0.f,-1.f,2.5f,1.5f};
+      assert(argmax_v(lo,8)==1 && "pick the +Inf position"); }
+    { float lo[8]; for(int i=0;i<8;i++) lo[i]=NAN;
+      assert(argmax_v(lo,8)==0 && "all-NaN: no crash, defined fallback"); }
+
+    g_temp=0.7f; g_nuc=0.9f;   /* the default serve/chat sampling path */
+
+    /* --- dist_build: a NaN logit must yield a finite one-hot, not all-NaN --- */
+    { float lo[8]={0.5f,1.f,NAN,8.f,0.2f,-1.f,0.f,0.3f};   /* max finite = idx3 (8.0) */
+      dist_build(lo,8);
+      double sum=0; int nan=0;
+      for(int i=0;i<8;i++){ if(!(g_pbuf[i]==g_pbuf[i])) nan=1; sum+=g_pbuf[i]; }
+      assert(!nan && "g_pbuf must be finite after a NaN logit");
+      assert(approx1(sum) && "g_pbuf must normalize to 1");
+      assert(approx1(g_pbuf[3]) && "mass must land on the max finite logit (idx3)");
+      assert(dist_sample(8,-1)==3 && "sampler emits the finite argmax, not token 0"); }
+
+    /* --- regression: clean logits still produce a valid distribution --- */
+    { float lo[8]={0.1f,0.2f,3.0f,0.4f,0.5f,0.6f,0.7f,0.8f};   /* peak = idx2 */
+      dist_build(lo,8);
+      double sum=0; int nan=0;
+      for(int i=0;i<8;i++){ if(!(g_pbuf[i]==g_pbuf[i])) nan=1; sum+=g_pbuf[i]; }
+      assert(!nan && "clean softmax stays finite");
+      assert(approx1(sum) && "clean softmax must sum to 1");
+      assert(g_pbuf[2]>=g_pbuf[0] && "peak token keeps the most mass"); }
+
+    printf("OK test_logit_nan: argmax_v NaN-skip + dist_build finite-collapse\n");
+    return 0;
+}