Scaffolding: ZMK module structure, Kconfig, DTS, behavior driver

- zephyr/module.yml with dts_root for dt-bindings
- Kconfig: STENO_ENGINE, dict selection (Plover/Lapwing/test),
  MPHF toggle, Unicode modes, history size, multi-stroke timeout
- DTS behavior binding (one_param, steno key index)
- behavior_steno.c: chord accumulation, all-up detection,
  multi-stroke buffering with timeout, star undo, formatter pipeline,
  3-way dict dispatch (split/MPHF/simple trie)
- dict_embed.S: .incbin from generated header path
- steno_keys.h: 23-key layout, bit positions matching compiler

tools/requirements.txt

@@ -0,0 +1,5 @@
+# zmk-steno-engine dictionary compiler
+# stdlib only — no external dependencies required
+#
+# Python >= 3.8
+# Uses: json, struct, zlib, argparse, sys, os, collections, math