Add mdview markdown viewer, reorganize tests/examples and libc layout

- Split tests/ (libc feature tests) and examples/ (real apps); shared
  app.mk in repo root, was examples/example.mk
- libc/io/* split into libc/{conio,env,errno,file,mouse,string,sys,
  time,video}/ — clearer module boundaries
- New examples/mdview/: markdown viewer (Phases 1-5 + light nested
  lists). Headers (H1-H4), HR, ulist/olist/quote with nesting via
  leading spaces, fenced code blocks, inline emphasis (bold/italic/
  underscore/code), wrap/unwrap mode with soft wrap (F2), horizontal
  pan (← →) with '>' truncation indicator
- libc additions: scroll() in conio (ESTEX SCROLL), strlwr/strupr,
  gets() test
- Makefile updates across tests/ for the new shared app.mk path

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

libc/include/fcntl.h

@@ -16,14 +16,22 @@
 #ifndef FCNTL_H
 #define FCNTL_H
 
-#define O_RDONLY  0
-#define O_WRONLY  1
-#define O_RDWR    2
+#ifndef	_STD_SEEK_
+#define	_STD_SEEK_
+/* constants to be used as 3rd argument for "fseek" function */
+#define	SEEK_SET	0
+#define	SEEK_CUR	1
+#define	SEEK_END	2
+#endif
 
-#define O_CREAT   0x040
-#define O_EXCL    0x080
-#define O_TRUNC   0x200
-#define O_APPEND  0x400
+/* Definition "open flags" */
+#define	O_WRONLY	0x01		/* 0  file write only */
+#define	O_RDONLY	0x02		/* 1  file read only */
+#define	O_RDWR		0x03		/* 1,0 file read/write */
+#define	O_TRUNC		0x04		/* 2  open with truncation */
+#define	O_CREAT		0x08		/* 3  create and open file */
+#define	O_EXCL		0x10		/* 4  exclusive open */
+#define	O_APPEND	0x20		/* 5  to end of file */
 
 int open (const char *path, int flags);
 int creat(const char *path, int mode);   /* mode arg ignored on Sprinter */