perf(protocol): skip string debug escaping when proto debug is off

output_escape() was called on every send_str/receive_str even when
LOG_DEBUG_PROTO logging was disabled, allocating and scanning the whole
payload for a line that log_debug_message() then discarded.  Add a
log_debug_enabled(flag) gate mirroring log_debug_message()'s own filter and
check it before escaping.  Redacted (secret) strings still log the same
<redacted> marker; no observable log output changes.

src/shared/log.h

@@ -29,6 +29,10 @@ void log_perror(const char* context);
 void set_log_level(LogLevel level);
 void set_log_debug_flags(uint32_t flags);
 uint32_t get_log_debug_flags(void);
+/* True when a log_debug_message() call with the same flag would actually emit:
+ * the debug log level is enabled AND the flag is selected.  Hot paths use this
+ * to skip expensive message formatting/escaping when the line is filtered. */
+bool log_debug_enabled(LogDebugFlag flag);
 void log_debug_message(LogDebugFlag flag, const char* message, ...);
 void set_log_info_flags(uint32_t flags);
 uint32_t get_log_info_flags(void);