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>

tests/stattest/Makefile

@@ -0,0 +1,3 @@
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := stattest
+include $(PROJ_ROOT)/app.mk

tests/stattest/stattest.c

@@ -0,0 +1,89 @@
+/*
+ * stattest — exercise POSIX stat() and fstat() over ESTEX file metadata.
+ *
+ * Calls:
+ *   stat() on a file by path
+ *   stat() on a directory (current dir)
+ *   stat() on a non-existent file (expects errno=ENOENT)
+ *   fstat() on a freshly opened file
+ */
+
+#include <stdio.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <time.h>
+
+static const char *mode_str(uint16_t m)
+{
+    if (S_ISDIR(m)) return "DIR";
+    if (S_ISREG(m)) return "REG";
+    return "???";
+}
+
+static void print_stat(const char *what, const struct stat *st)
+{
+    char buf[32];
+    /* Reuse asctime by converting epoch → struct tm. */
+    time_t t = st->st_mtime;
+    struct tm *tm = localtime(&t);
+    sprintf(buf, "%04d-%02d-%02d %02d:%02d:%02d",
+            tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday,
+            tm->tm_hour, tm->tm_min, tm->tm_sec);
+
+    printf("  %s: mode=%s perm=0%o size=%lu mtime=%s\n",
+           what,
+           mode_str(st->st_mode),
+           (unsigned)(st->st_mode & 0777),
+           (unsigned long)st->st_size,
+           buf);
+}
+
+int main(int argc, char **argv)
+{
+    struct stat st;
+
+    puts("stattest: stat() / fstat() smoke test");
+
+    /* stat on this very file (the .exe). */
+    if (stat(argv[0], &st) == 0)
+        print_stat("self.exe", &st);
+    else
+        printf("  stat(\"%s\") failed: errno=%d (%s)\n",
+               argv[0], errno, strerror(errno));
+
+    /* stat on a definitely-missing file. */
+    errno = 0;
+    if (stat("NOSUCH.XXX", &st) == 0)
+        puts("  stat(NOSUCH.XXX) unexpectedly succeeded?!");
+    else
+        printf("  stat(NOSUCH.XXX) -> errno=%d (%s) [expected ENOENT]\n",
+               errno, strerror(errno));
+
+    /* fstat on a fresh handle (open the running exe read-only). */
+    int fd = open(argv[0], O_RDONLY);
+    if (fd < 0) {
+        printf("  open(\"%s\") failed: errno=%d (%s)\n",
+               argv[0], errno, strerror(errno));
+    } else {
+        if (fstat(fd, &st) == 0)
+            print_stat("fstat(self)", &st);
+        else
+            printf("  fstat failed: errno=%d (%s)\n",
+                   errno, strerror(errno));
+        close(fd);
+    }
+
+    /* Extra argv-passed path. */
+    if (argc > 1) {
+        if (stat(argv[1], &st) == 0)
+            print_stat(argv[1], &st);
+        else
+            printf("  stat(\"%s\") failed: errno=%d (%s)\n",
+                   argv[1], errno, strerror(errno));
+    }
+
+    return 0;
+}