Add CSV and JSONL statistics output

[m4r1k]

Wireblast's terminal output is useful while a run is active, but benchmark
automation should not have to parse display text. This adds optional
machine-readable statistics to --no-tui.

--stats-file PATH creates a new output file and refuses to overwrite an
existing one. CSV is the default format; --stats-format jsonl selects
newline-delimited JSON. Each one-second sample is flushed immediately, so an
interrupted run still leaves usable records. Clean shutdown adds a final
aggregate and one record for every AF_XDP queue.

Every record has a schema version, UTC timestamp, elapsed time, traffic totals
and rates, and the Linux XDP_STATISTICS counters. The schema distinguishes
Wireblast packet totals from AF_XDP ring accounting:
kernel_rx_descriptors and kernel_tx_descriptors report descriptor progress,
not packet counts. A multi-buffer packet can occupy more than one descriptor.
The six AF_XDP counter fields use their Linux UAPI names.

The existing terminal output is unchanged. Statistics paths and formats belong
to one invocation and are not saved in wizard preferences. The patch adds no
dependencies.

The CSV path was also exercised between paired Google Cloud C4N instances from
c4n-highcpu-2 through c4n-highcpu-96. Every admitted run used gVNIC with the
GVE driver, native XDP, and AF_XDP zero-copy. The inline CSV sample is derived
from a C96 IMIX run; its records come from the live capture and its four
counter headers have been updated to the final schema names used by this PR.
JSONL was covered by the exporter and CLI tests.

C96 gVNIC/GVE CSV sample

schema_version,record_type,timestamp_utc,elapsed_seconds,state,transmits,queue_id,tx_packets,tx_bytes,tx_udp,tx_tcp,tx_other,tx_errors,tx_pps,tx_l1_bps,tx_l2_bps,rx_packets,rx_bytes,rx_udp,rx_tcp,rx_other,rx_drops,rx_pps,rx_l1_bps,rx_l2_bps,kernel_queues,kernel_rx_descriptors,kernel_tx_descriptors,rx_dropped,rx_ring_full,rx_fill_ring_empty_descs,rx_invalid_descs,tx_invalid_descs,tx_ring_empty_descs
1,sample,2026-08-10T23:09:51.699887839Z,59.751228,running,false,,0,0,0,0,0,0,0.000000,0.000000,0.000000,1670659716,607644031628,1670659716,0,0,0,47987729.749624,147217033254.869446,139538996494.929657,32,1670662489,0,0,0,1916,0,0,104997015
1,final,2026-08-10T23:10:39.453856667Z,107.505197,complete,false,,0,0,0,0,0,0,0.000000,0.000000,0.000000,2885451046,1049518523936,2885451046,0,0,0,0.000000,0.000000,0.000000,32,2885451046,0,0,0,2408,0,0,181705415

Tested with:

• go test -count=1 ./...
• go test -race -count=1 ./...
• go vet ./...
• gofmt -l .
• go build -buildvcs=false ./cmd/wireblast