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).

include/dt-bindings/zmk/javelin_steno.h

@@ -0,0 +1,41 @@
+// Steno key indices matching Javelin's StenoKey enum.
+// Use in keymap: &javsteno STENO_S1, &javsteno STENO_TL, etc.
+
+#define STENO_S1    0
+#define STENO_S2    1
+#define STENO_TL    2
+#define STENO_KL    3
+#define STENO_PL    4
+#define STENO_WL    5
+#define STENO_HL    6
+#define STENO_RL    7
+#define STENO_A     8
+#define STENO_O     9
+#define STENO_STAR1 10
+#define STENO_STAR2 11
+#define STENO_STAR3 12
+#define STENO_STAR4 13
+#define STENO_E     14
+#define STENO_U     15
+#define STENO_FR    16
+#define STENO_RR    17
+#define STENO_PR    18
+#define STENO_BR    19
+#define STENO_LR    20
+#define STENO_GR    21
+#define STENO_TR    22
+#define STENO_SR    23
+#define STENO_DR    24
+#define STENO_ZR    25
+#define STENO_NUM1  26
+#define STENO_NUM2  27
+#define STENO_NUM3  28
+#define STENO_NUM4  29
+#define STENO_NUM5  30
+#define STENO_NUM6  31
+#define STENO_NUM7  32
+#define STENO_NUM8  33
+#define STENO_NUM9  34
+#define STENO_NUM10 35
+#define STENO_NUM11 36
+#define STENO_NUM12 37

include/zmk_javelin_steno/zmk_platform_shim.h

@@ -9,4 +9,12 @@ void zmk_javelin_steno_process_key(int steno_key_index, bool is_press);
 
 #ifdef __cplusplus
 }
+
+class StenoDictionary;
+struct StenoOrthography;
+
+// Complete engine initialization with loaded dictionary.
+// Called from dictionary loading code once flash data is validated.
+void zmk_javelin_steno_init_engine(StenoDictionary &dictionary,
+                                   const StenoOrthography &orthography);
 #endif