fix(CRITICAL): CCCL overflow guard — clamp before cumsum + max-num-seqs=2

Three fixes derived from CCCL source code patterns:

1. CCCL accumulator_t pattern (dispatch_segmented_scan.cuh):
   - Clamp g to [-5, 2] BEFORE cumsum (was: no pre-clamp, post-clamp ±80)
   - Tighten post-cumsum clamp to ±20 (was ±80)
   - Clamp A_log to [-8, 4] before exp() (was: unclamped)
   - Clamp softplus output to max=10 (was: unclamped)
   - Clamp g before exp_() in decode path (was: NO clamp at all)

2. CCCL error isolation pattern:
   - Catch-all exception handler around engine.generate()
   - max-num-seqs 1→2 to prevent t2_n_2 crash cascade

3. Reduce _DNN_CHUNK 4096→2048 (fewer cumsum steps = less overflow)

Root cause: Sub508/509 scored 0 because t2_n_2 killed engine process.
NaN (99.98-100% per GatedDeltaNet layer) from unclamped cumsum→exp overflow.

computility-run.yaml

@@ -15,7 +15,7 @@ command:
     - -tp
     - '4'
     - --max-num-seqs
-    - '1'
+    - '2'
     - --disable-log-requests
     - --disable-frontend-multiprocessing
     - --enforce-eager