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/cat/Makefile

@@ -0,0 +1,6 @@
+# Build cat.exe — uses lib/sprinter.lib in TINY memory mode.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := cat
+EXTRA_DATA := test.txt
+include $(PROJ_ROOT)/app.mk

tests/cat/cat.c

@@ -0,0 +1,31 @@
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+int main(void)
+{
+    int fd = open("TEST.TXT", O_RDONLY);
+    if (fd < 0) {
+        puts("cat: cannot open TEST.TXT");
+        puts("Press any key to exit...");
+        (void)getchar();
+        return 1;
+    }
+
+    puts("--- TEST.TXT ---");
+
+    char buf[64];
+    int n;
+    while ((n = read(fd, buf, sizeof(buf))) > 0) {
+        for (int i = 0; i < n; i++) {
+            putchar((unsigned char)buf[i]);
+        }
+    }
+
+    close(fd);
+
+    puts("");
+    puts("--- end of file. Press any key to exit. ---");
+    (void)getchar();
+    return 0;
+}

tests/cat/test.txt

@@ -0,0 +1,13 @@
+Hello from a file on disk!
+This text was read by cat.exe through
+the ESTEX file I/O syscalls:
+  open  ($11)
+  read  ($13)
+  close ($12)
+
+The whole loop hits real DSS code on
+the Sprinter (in MAME, just an image).
+
+Line 9.
+Line 10.
+Final line.