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/stest2/Makefile

@@ -0,0 +1,3 @@
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := stest2
+include $(PROJ_ROOT)/app.mk

tests/stest2/stest2.c

@@ -0,0 +1,43 @@
+/*
+ * 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>
+#include <dir.h>
+
+
+int main(int argc, char **argv)
+{
+    struct stat st;
+
+    puts("stattest: stat() / fstat() smoke test");
+
+    ffblk_t ffb;
+
+    if (ffirst(argv[0], &ffb, FA_DIREC) == 0) {
+        printf("File %s, size = %d\n", argv[0], ffb.size);
+    } else {
+        printf("File %s, error\n", argv[0]);
+    }
+
+    if (argc > 1) {
+        if (ffirst(argv[1], &ffb, FA_DIREC) == 0) {
+            printf("File %s, size = %d\n", argv[1], ffb.size);
+        } else {
+            printf("File %s, error\n", argv[1]);
+        }
+    }
+    return 0;
+}