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/dec_test/Makefile

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

tests/dec_test/dec_test.c

@@ -0,0 +1,39 @@
+#include <stdio.h>
+extern void dec8 (unsigned char);
+extern void dec16(unsigned int);
+extern void dec32(unsigned long);
+extern void hex8 (unsigned char);
+extern void hex16(unsigned int);
+extern void hex32(unsigned long);
+
+int main(void) {
+    puts("hex8:");
+    hex8(0x00); puts("");
+    hex8(0x5A); puts("");
+    hex8(0xFF); puts("");
+    puts("hex16:");
+    hex16(0x0000); puts("");
+    hex16(0xCAFE); puts("");
+    hex16(0xFFFF); puts("");
+    puts("hex32:");
+    hex32(0x00000000UL); puts("");
+    hex32(0xDEADBEEFUL); puts("");
+    hex32(0xFFFFFFFFUL); puts("");
+    puts("dec8:");
+    dec8(0);   puts("");
+    dec8(42);  puts("");
+    dec8(255); puts("");
+    puts("dec16:");
+    dec16(0);     puts("");
+    dec16(1234);  puts("");
+    dec16(65535); puts("");
+    puts("dec32:");
+    dec32(0UL);          puts("");
+    dec32(98765UL);      puts("");
+    dec32(123456UL);     puts("");
+    dec32(123456789UL);  puts("");
+    dec32(4294967295UL); puts("");
+    puts("done — any key");
+    (void)getchar();
+    return 0;
+}