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.

tests/profiler/behave_shell/test_calibration_grid.py

@@ -109,6 +109,7 @@ PHASE_G_CONDITIONAL_PRIMITIVES: frozenset[str] = frozenset({
     "emotional_valence.arousal",            # needs typing bursts
     "emotional_valence.valence",            # needs ≥ 80 typed letters
     "emotional_valence.frustration_venting",  # needs ≥ 30 typed letters
+    "motor.digraph_simhash",                # needs ≥ 8 distinct digraphs / ≥ 20 samples
 })
 
 # Backwards-compatible aliases for any external import — earlier phases