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/profiler/behave_shell/_features/__init__.py

@@ -52,6 +52,7 @@ from decnet.profiler.behave_shell._features.temporal import (
 )
 from decnet.profiler.behave_shell._features.motor import (
     command_chunking,
+    digraph_simhash,
     error_correction,
     input_modality,
     keystroke_cadence,
@@ -68,6 +69,7 @@ FEATURES: tuple[FeatureFn, ...] = (
     input_modality,
     paste_burst_rate,
     keystroke_cadence,
+    digraph_simhash,
     motor_stability,
     error_correction,
     command_chunking,