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/sys/atexit.c

@@ -0,0 +1,57 @@
+/*
+ * atexit + exit + _exit.
+ *
+ *   atexit(fn)  — register fn to be called at normal termination (max 8).
+ *   exit(code)  — run the atexit chain in LIFO, then ESTEX EXIT.
+ *   _exit(code) — POSIX raw exit: skip the chain, go straight to ESTEX.
+ *
+ * These functions own termination entirely; crt0.s only does inline RST
+ * 10h #41 when main returns without an explicit exit().  That path skips
+ * the atexit chain — programs that need handlers should call exit() at
+ * the bottom of main (or return through a wrapper).
+ */
+
+#include <stdlib.h>
+#include <sprinter_exit.h>
+
+#define ATEXIT_MAX  8
+
+static void (*atexit_stack[ATEXIT_MAX])(void);
+static int   atexit_top = 0;
+
+int atexit(void (*fn)(void))
+{
+    if (atexit_top >= ATEXIT_MAX) {
+        return -1;
+    }
+    atexit_stack[atexit_top++] = fn;
+    return 0;
+}
+
+/* exit() — runs the chain, then performs the raw ESTEX EXIT. */
+void exit(int code)
+{
+    while (atexit_top > 0) {
+        void (*fn)(void) = atexit_stack[--atexit_top];
+        if (fn) {
+            fn();
+        }
+    }
+    _exit(code);   /* falls into the inline-asm raw exit below */
+}
+
+/* _exit() — POSIX raw termination, no atexit chain. */
+void _exit(int code) __naked
+{
+    (void)code;
+    __asm
+        ;; HL = code (single int arg).
+        ld    a, l
+        ld    b, a
+        ld    c, #0x41        ; ESTEX EXIT
+        rst   #0x10
+        ;; Should not return.
+1$:     halt
+        jr    1$
+    __endasm;
+}