Clear the whole stage buffer in the mesh fallback too, before the mesh program runs.

On a GPU without mesh shaders the fallback replaces the translated module's entry function with its own, which cleared
only the primitive count and then copied the whole slot (vertex lines, count, index bytes) out of workgroup memory to
be drawn. Lines and index bytes the program did not write kept what an earlier workgroup left there and were drawn as
vertices and primitives. The fallback's entry function now sets every word of the slot to zero in a loop over the
workgroup's threads before the payload copy and the barrier, as the translator's own entry function does.

Checked: --probe-meshfb passes before and after (the fallback's pictures match the mesh pipeline's texel for texel on
an RTX GPU); its module part converts all 520 task and mesh programs of an installed shader cache with 0 failures, all
1,020 resulting modules pass spirv-val (vulkan1.3) and their 520 compute pipelines build on the device.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/mesh_fb.c

@@ -376,10 +376,14 @@ static uint32_t *convert(const uint32_t *w, uint64_t words, const MeshFbInfo *m,
         DO(&c, OpStore, fake_wg, E(&c, OpCompositeConstruct, c.t_v3uint, j, k0, k0));
         uint32_t at = mul(&c, g, K(&c, slot_words));
         uint32_t room = E(&c, OpULessThanEqual, c.t_bool, add(&c, at, K(&c, slot_words)), E(&c, OpArrayLength, c.t_uint, mesh_buf, 0));
-        // the primitive count starts at 0: a mesh program that emits nothing never writes it
-        uint32_t mg = if_begin(&c, first);
-        DO(&c, OpStore, SHARED(count_word), k0);
-        if_end(&c, mg);
+        // The whole stage buffer starts at 0: the vertex lines, the primitive count and the vertex indices. Workgroup
+        // memory is undefined at the start, a mesh program need not write every vertex it has room for or every index
+        // byte (nor its count when it emits nothing), and the slot below is copied whole and drawn: what an earlier
+        // workgroup left there became vertices and primitives. The payload area after it is copied in next.
+        Loop z = loop_begin(&c, counter, tid, K(&c, slot_words), threads);
+        DO(&c, OpStore, SHARED(z.i), k0);
+        loop_end(&c, &z);
+        uint32_t mg;
         if (m->has_task) {
             uint32_t pw = m->payload_bytes / 4;
             if (pw > task_words) pw = task_words;