fix(nvidia): surface Xid 79/154 GPU bus-fall-off as a physical-reboot-required signal

nvidia-bug-report.sh appends .gz to --output-file when gzip is available,
which silently produced empty nvidia-bug-report.txt in support bundles
(cat looked for the uncompressed name that never existed).

Also: a GPU that falls off the PCIe/NVLink bus (Xid 79) or gets flagged
for Node Reboot Required (Xid 154) mid-SAT-run left every downstream test
failing with generic, unrelated-looking errors (CUDA "unknown error",
"unable to determine device handle") with no indication the GPU needed a
physical power-cycle to recover. Detect these codes from SAT run logs and
surface a plain-English "physical reboot required" message in the task's
failure detail, the persisted component-status DB, a dashboard banner on
the Hardware Summary card, and topology diagram GPU-node severity.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/collector/xid_test.go

@@ -28,9 +28,10 @@ func TestXidSeverity(t *testing.T) {
 			wantOK:  true,
 		},
 		{
-			name:   "xid 79 unknown code falls back to caller default",
-			line:   "NVRM: Xid (PCI:0000:65:00): 79, pid=1234, GPU has fallen off the bus",
-			wantOK: false,
+			name:    "xid 79 GPU fell off the bus is critical",
+			line:    "NVRM: Xid (PCI:0000:65:00): 79, pid=1234, GPU has fallen off the bus",
+			wantSev: "critical",
+			wantOK:  true,
 		},
 		{
 			name:   "no xid in line",