Measure a regroup on the tablet, not just on the desktop

The GPU question needed a number nobody had: how a regroup divides on
the hardware whose CPU is weakest. dr-face carries no weights and
touches no display, and dr-catalog's example needs only a catalog file,
so both run under adb shell against a copy of a real library.

On the same 18,143 faces — desktop against the tablet — scan 0.96s /
2.61s, agglomerate 1.69s / 2.16s, score 0.26s / 0.40s. The scan is half
the pass on the tablet and under a third on the desktop, because twenty
cores of AVX2 pull ahead of NEON much further than the merge engine's
single-threaded hashing does. So a GPU GEMM is worth roughly 2× a
regroup on the tablet and 1.5× here, and it is the tablet that should
decide whether it is built.

The two architectures agree exactly: the same 1,531,969 evidence pairs,
the same 2,518 groups holding the same 16,246 faces, the same
reliability table. That is a better check on the NEON kernel than the
unit test can be.

Two instruments, both read-only: the example now prints its phases, and
dr-face gains scan_bench, which needs no library at all and so can
answer "how fast is this machine" on a device with nothing on it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/traceability.md

@@ -9,7 +9,7 @@ Denominators are parsed from [`requirements.md`](requirements.md) at run time, n
 
 | Metric | Value |
 |---|---|
-| Source files scanned | 280 |
+| Source files scanned | 281 |
 | TRACES tags found | 812 |
 | Requirements defined | 177 |
 | Requirements covered | 106 |