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/conio.h

@@ -38,6 +38,42 @@
 char kbhit (void);
 char getch (void);
 char getche(void);
+
+/* Extended-key reader. Returns (scan << 8) | ascii.  Plain ASCII keys
+ * have ascii in the low byte; extended keys (arrows / F1..F12 /
+ * PgUp/PgDn / Home / End / Ins / Del) carry ascii == 0 and the
+ * KEY_* code in the high byte. */
+uint16_t getkey(void);
+
+/* Scan codes returned in the high byte of getkey() when the low byte
+ * (ASCII) is 0.  Empirically verified in MAME — the ProgrammerManual.txt
+ * "positional code" column is misleading; BIOS returns IBM-style codes
+ * for the F-keys and a "5N + numpad-position" pattern for the cursor /
+ * editing keys.
+ *
+ * Verified 2026-06-04 by reading raw getkey() output. */
+#define KEY_F1     0x3B
+#define KEY_F2     0x3C
+#define KEY_F3     0x3D
+#define KEY_F4     0x3E
+#define KEY_F5     0x3F
+#define KEY_F6     0x40
+#define KEY_F7     0x41
+#define KEY_F8     0x42
+#define KEY_F9     0x43
+#define KEY_F10    0x44
+#define KEY_F11    0x45   /* not verified */
+#define KEY_F12    0x46   /* not verified */
+#define KEY_END    0x51
+#define KEY_DOWN   0x52
+#define KEY_PGDN   0x53
+#define KEY_LEFT   0x54
+#define KEY_RIGHT  0x56
+#define KEY_HOME   0x57
+#define KEY_UP     0x58
+#define KEY_PGUP   0x59
+#define KEY_INS    0x50   /* numpad 0; not verified */
+#define KEY_DEL    0x55   /* numpad 5/.; not verified */
 char putch (char c);
 char cputs (const char *s);
 int  cprintf(const char *fmt, ...);
@@ -63,6 +99,8 @@ uint8_t  wherex (void);
 uint8_t  wherey (void);
 uint16_t wherexy(void); // high byte = Y, low byte = X coords.
 
+void scroll(uint8_t, uint8_t, uint8_t, uint8_t, uint8_t, uint8_t);
+
 /* Direct character/attribute screen access (ESTEX $57 / $58).
  *   wrchar — write char + attribute at (x, y); does NOT advance the cursor
  *            and does NOT interpret control characters.  Useful for
@@ -143,10 +181,22 @@ uint8_t get_putch_raw_mode(void);
  * Colour order is standard CGA / Borland-conio.h.  Constants 0..7 are
  * usable for both fg and bg; 8..15 are foreground-only. */
 enum {
-    COLOR_BLACK = 0, COLOR_BLUE,  COLOR_GREEN,   COLOR_CYAN,
-    COLOR_RED,       COLOR_MAGENTA, COLOR_BROWN, COLOR_LIGHTGRAY,
-    COLOR_DARKGRAY,  COLOR_LIGHTBLUE, COLOR_LIGHTGREEN, COLOR_LIGHTCYAN,
-    COLOR_LIGHTRED,  COLOR_LIGHTMAGENTA, COLOR_YELLOW, COLOR_WHITE
+    COLOR_BLACK = 0, 
+    COLOR_BLUE,  
+    COLOR_GREEN,   
+    COLOR_CYAN,
+    COLOR_RED,       
+    COLOR_MAGENTA, 
+    COLOR_BROWN, 
+    COLOR_LIGHTGRAY,
+    COLOR_DARKGRAY,  
+    COLOR_LIGHTBLUE, 
+    COLOR_LIGHTGREEN, 
+    COLOR_LIGHTCYAN,
+    COLOR_LIGHTRED,  
+    COLOR_LIGHTMAGENTA, 
+    COLOR_YELLOW, 
+    COLOR_WHITE
 };
 #define COLOR_BLINK    0x80u
 #define COLOR(fg, bg)  ((uint8_t)((((bg) & 0x07) << 4) | ((fg) & 0x0F)))