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/stdlib/Makefile

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

tests/stdlib/stdlib.c

@@ -0,0 +1,63 @@
+/*
+ * stdlib_test — verifies that SDCC's z80.lib stdlib functions (atoi,
+ * strtol, rand, qsort, bsearch, abs, ldiv) work in our environment.
+ * If they all behave correctly, we can save ourselves the labour of
+ * reimplementing them in libc/stdlib.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static int cmp_int(const void *a, const void *b)
+{
+    int ia = *(const int *)a;
+    int ib = *(const int *)b;
+    return (ia > ib) - (ia < ib);
+}
+
+int main(int argc, char **argv)
+{
+    /* atoi / strtol */
+    int       v_atoi  = atoi("12345");
+    long      v_strtol = strtol("-9876", NULL, 10);
+    long      v_hex    = strtol("ff", NULL, 16);
+    printf("atoi(\"12345\") = %d (expected 12345)\n", v_atoi);
+    printf("strtol(\"-9876\",,10) = %ld (expected -9876)\n", v_strtol);
+    printf("strtol(\"ff\",,16) = %ld (expected 255)\n", v_hex);
+
+    /* abs / ldiv */
+    printf("abs(-7) = %d (expected 7)\n", abs(-7));
+    {
+        ldiv_t q = ldiv(100L, 7L);
+        printf("ldiv(100,7) = {q=%ld, r=%ld} (expected {14, 2})\n",
+               q.quot, q.rem);
+    }
+
+    /* rand / srand — deterministic with fixed seed */
+    srand(42);
+    printf("rand x3 (seed 42): %d %d %d\n", rand(), rand(), rand());
+
+    /* qsort */
+    {
+        int arr[] = { 5, 1, 4, 2, 8, 3, 7, 6, 0, 9 };
+        qsort(arr, 10, sizeof(int), cmp_int);
+        printf("qsort:");
+        for (int i = 0; i < 10; i++) printf(" %d", arr[i]);
+        printf(" (expected 0..9 sorted)\n");
+
+        /* bsearch */
+        int key = 7;
+        int *p = (int *)bsearch(&key, arr, 10, sizeof(int), cmp_int);
+        printf("bsearch(7) -> %s (expected found)\n",
+               (p && *p == 7) ? "found" : "MISS");
+    }
+
+    /* argv-as-int: typical CLI use */
+    if (argc > 1) {
+        printf("argv[1] as int: %d\n", atoi(argv[1]));
+    }
+
+    puts("\nall SDCC stdlib functions reachable.");
+    return 0;
+}