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/string/strlwr.c

@@ -0,0 +1,23 @@
+/*
+ * solid_compat.c — Solid-C compatibility helpers that need real code
+ * (rather than just header macros).
+ *
+ * CP866 Cyrillic support: strlwr/strupr handle uppercase/lowercase
+ * conversion for both Latin and Cyrillic characters in CP866 code page
+ * (bytes 0x80–0xFF).
+ */
+
+#include <sprinter_compat.h>
+#include <ctype.h>
+
+char *strlwr(char *s)
+{
+    char *p = s;
+    while (*p) {
+        if ((*p >= 'A' && *p <= 'Z') || (*p >= 0x80 && *p <= 0x8F)) *p += 'a' - 'A';
+        else if ((*p >= 0x90 && *p <= 0x9F)) *p += 0x50;
+        else if ((*p == 0xF0)) *p = 0xF1;
+        p++;
+    }
+    return s;
+}