feat(profiler): extract motor.digraph_simhash keystroke biometric

Per-session 64-bit SimHash of inter-keystroke digraph flight times:
walk single-char input events, accumulate flight time per (c1,c2),
bucket the median, Charikar-SimHash the bucketed pairs. Locality-
sensitive so the same typist is Hamming-close across sessions; pastes
and think-pauses break the chain; silent below the sample-size floor.

New shared decnet/util/simhash.py (simhash64/hamming64/bytes helpers).
Registered as a conditional Tier-A primitive (count 37->38); requires
behave-shell>=0.1.2.

decnet/util/__init__.py

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Small cross-cutting helpers with no domain home of their own."""