Phase 3: dictionary loading pipeline + binary embedding

- engine_init.cc: loads StenoDictionaryCollection from embedded binary,
  validates JSC4 magic + timestamp, constructs dict list + full processor
  pipeline (Repeat → AllUp → JeffModifiers → Engine)
- dict_embed.S: embeds compiled dict binary via .incbin directive.
  Falls back to null placeholder if no dict available
- CMakeLists.txt: dict binary discovery (dicts/steno_dict.bin or
  JAVELIN_DICT_BIN cmake var), assembly embedding
- tools/dict_compiler/compile_dict.py: stroke parser + skeleton compiler.
  Full JSC4 binary format not yet implemented — for now, use Javelin
  web tool (lim.au) to generate dict binary
- dicts/: directory for compiled dictionary binaries

Dict compiler is the remaining gap — JSC4 format requires exact XIP
pointer layout matching Javelin's C++ struct casting.

include/zmk_javelin_steno/zmk_platform_shim.h

@@ -7,14 +7,13 @@ extern "C" {
 void zmk_javelin_steno_init(void);
 void zmk_javelin_steno_process_key(int steno_key_index, bool is_press);
 
+// Linker symbols for the embedded dictionary binary
+extern const uint8_t _javelin_dict_start[];
+extern const uint8_t _javelin_dict_end[];
+
 #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