feat(profiler/behave_shell): emit environmental.keyboard_layout

ANTI authorised dropping the PII boundary for this primitive. ctx
gains typed_unigram_counts / typed_bigram_counts / typed_letter_count
populated during the existing single-pass input walk (paste-class
events excluded).

Two-axis classifier:
* layout-artefact unigrams take priority — q rate above floor with
  low English saturation → azerty; z above floor with y below → qwertz
* fallback to English-bigram saturation: ≥ floor → qwerty, else other

Sample-size floor 200 typed letters; bigram histogram capped at
top-64 to bound memory. Confidence cap stays moderate (0.40-0.55) —
heuristic discriminator.

decnet/profiler/behave_shell/_features/__init__.py

@@ -25,6 +25,7 @@ from decnet.profiler.behave_shell._features.cognitive import (
     inter_command_latency_class,
 )
 from decnet.profiler.behave_shell._features.environmental import (
+    keyboard_layout,
     locale,
     shell_type,
     terminal_multiplexer,
@@ -75,4 +76,5 @@ FEATURES: tuple[FeatureFn, ...] = (
     shell_type,
     terminal_multiplexer,
     locale,
+    keyboard_layout,
 )