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/strtest/strtest.c

@@ -0,0 +1,53 @@
+#include <stdio.h>
+#include <string.h>
+
+static void check(const char *label, int ok)
+{
+    puts(label);
+    puts(ok ? "  OK" : "  FAIL");
+}
+
+int main(void)
+{
+    char buf[32];
+
+    /* strlen */
+    check("strlen(\"\") == 0",         strlen("") == 0);
+    check("strlen(\"hello\") == 5",    strlen("hello") == 5);
+
+    /* strcpy + strlen */
+    strcpy(buf, "Sprinter");
+    check("strcpy + strlen == 8",      strlen(buf) == 8);
+
+    /* strcmp */
+    check("strcmp(\"a\",\"a\") == 0",  strcmp("a", "a") == 0);
+    check("strcmp(\"a\",\"b\") < 0",   strcmp("a", "b") < 0);
+    check("strcmp(\"b\",\"a\") > 0",   strcmp("b", "a") > 0);
+    check("strcmp(buf,\"Sprinter\")",  strcmp(buf, "Sprinter") == 0);
+
+    /* strcat */
+    strcpy(buf, "Hello ");
+    strcat(buf, "Sprinter!");
+    check("strcat -> \"Hello Sprinter!\"", strcmp(buf, "Hello Sprinter!") == 0);
+
+    /* memcpy */
+    memset(buf, 'X', 16);
+    memcpy(buf + 4, "ABCD", 4);
+    buf[16] = 0;
+    check("memset+memcpy -> \"XXXXABCDXXXXXXXX\"",
+          strcmp(buf, "XXXXABCDXXXXXXXX") == 0);
+
+    /* memset */
+    memset(buf, 'q', 5);
+    buf[5] = 0;
+    check("memset(buf,'q',5) -> \"qqqqq\"", strcmp(buf, "qqqqq") == 0);
+
+    /* memcmp */
+    check("memcmp equal",   memcmp("abcd", "abcd", 4) == 0);
+    check("memcmp diff",    memcmp("abcd", "abxd", 4) != 0);
+
+    puts("");
+    puts("All checks done.  Press any key to exit.");
+    (void)getchar();
+    return 0;
+}