feat(stk.h, module, stk): improve dependency failure logging

- Add STK_MOD_DEP_LOG_BUFFER (2048) to stk.h for the dep failure message
buffer size.

- Add stk_log_dependency_failures(index, action) to module.c. Walks all
deps for the given module, skips satisfied ones, and builds a single
log line listing every unmet dep with its reason: "not found" or
"requires <constraint>, have <version>". The action parameter
("Deferring" / "Unloading") lets call sites produce contextually
appropriate messages:

  - Deferring 'test_mod_dep': unmet deps: test_mod (not found)
  - Unloading 'test_mod_dep': unmet deps: test_mod (not found), renderer (requires ^2.0.0, have 1.3.0)

- Replace the silent defer at init and the vague "unmet dependencies"
cascade log in stk_poll() with calls to stk_log_dependency_failures().

include/stk.h

@@ -7,6 +7,7 @@
 /* Buffers */
 #define STK_LOG_PREFIX_BUFFER 64
 #define STK_MOD_DEP_OPERATOR_BUFFER 3
+#define STK_MOD_DEP_LOG_BUFFER 2048
 #define STK_MOD_DESC_BUFFER 256
 #define STK_MOD_DIR_BUFFER 256
 #define STK_MOD_ID_BUFFER 64