fix(hwfit): add GB10 unified-memory bandwidth so speed scores are real (#4270)

NVIDIA Grace Blackwell GB10 / DGX Spark was missing from GPU_BANDWIDTH, so
_lookup_bandwidth() returned None for it and _estimate_speed() fell through
to the crude FALLBACK_K path (k/active-params). That over-stated tok/s and
let speed scores saturate regardless of the box's real ~273 GB/s LPDDR5X
pool — distorting model ranking on these 128GB unified-memory rigs.

Add "gb10": 273 (GB/s). nvidia-smi reports the device name as "NVIDIA GB10",
which substring-matches the new key, so detected GB10 boxes now estimate
speed from the real bandwidth instead of the fallback.

services/hwfit/fit.py

@@ -19,6 +19,10 @@ GPU_BANDWIDTH = {
     "6950 xt": 576, "6900 xt": 512, "6800 xt": 512, "6800": 512, "6700 xt": 384, "6600 xt": 256, "6600": 224,
     "mi300x": 5300, "mi300": 5300, "mi250x": 3277, "mi250": 3277, "mi210": 1638, "mi100": 1229,
     "9070 xt": 624, "9070": 488, "9060 xt": 322, "9060": 322,
+    # NVIDIA GB10 Grace-Blackwell superchip (DGX Spark). Unified LPDDR5X memory,
+    # not Apple Silicon, so it lives in the generic GPU table — the Apple-only
+    # lookup never matches it (its name carries no "apple").
+    "gb10": 273,
 }
 
 # Pre-sort keys by length descending for correct substring matching