Implement Phase 2: real platform shims + behavior driver + steno key defines

- zmk_key_shim: Key::Press/Release → raise_zmk_keycode_state_changed()
  Maps Javelin KeyCode to ZMK HID events (keyboard + consumer pages)
- zmk_clock_shim: Clock:: → Zephyr k_uptime_get/k_cyc_to_us/k_msleep
- zmk_flash_shim: Flash::EraseBlock/WriteBlock → Zephyr flash API
- behavior_javelin_steno.c: proper DT_DRV_COMPAT, DT_INST_FOREACH_STATUS_OKAY
- engine_init.cc: processor pipeline wiring (AllUp → JeffModifiers → Engine)
- javelin_steno.dtsi: default &javsteno behavior instance
- dt-bindings/zmk/javelin_steno.h: STENO_S1..STENO_ZR key defines for keymaps
- Exclude thread.cc (pthreads), key.cc, clock.cc (replaced by shims)
- Fix CMake: combine COMPILE_FLAGS + COMPILE_DEFINITIONS in single call

Engine init deferred until dictionary loaded (Phase 3).

CMakeLists.txt

@@ -14,7 +14,6 @@ if(CONFIG_ZMK_JAVELIN_STENO)
       javelin/bit.cc
       javelin/button_script_manager.cc
       javelin/button_script.cc
-      javelin/clock.cc
       javelin/console_input_buffer.cc
       javelin/console.cc
       javelin/container/cyclic_queue.cc
@@ -31,7 +30,6 @@ if(CONFIG_ZMK_JAVELIN_STENO)
       javelin/host_layout.cc
       javelin/key_code.cc
       javelin/key_press_parser.cc
-      javelin/key.cc
       javelin/mem.cc
       javelin/orthography.cc
       javelin/pattern_component.cc
@@ -55,7 +53,6 @@ if(CONFIG_ZMK_JAVELIN_STENO)
       javelin/stroke_list_parser.cc
       javelin/stroke.cc
       javelin/system.cc
-      javelin/thread.cc
       javelin/timer_manager.cc
       javelin/unicode_script.cc
       javelin/unicode.cc
@@ -182,13 +179,9 @@ if(CONFIG_ZMK_JAVELIN_STENO)
       ${ZMK_INIT_SRC}
     )
 
-    # Set C++ compile flags for all Javelin + shim sources
+    # C++ flags + Javelin compile definitions
     set_source_files_properties(${ALL_CXX_SRC} PROPERTIES
       COMPILE_FLAGS "-std=gnu++23 -fno-exceptions -fno-rtti -fno-threadsafe-statics"
-    )
-
-    # Compile definitions for Javelin
-    set_source_files_properties(${ALL_CXX_SRC} PROPERTIES
       COMPILE_DEFINITIONS "JAVELIN_USE_EMBEDDED_STENO=1;JAVELIN_USE_USER_DICTIONARY=1;JAVELIN_PLATFORM_ZEPHYR=1"
     )