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/bankedbg/bank2.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <sprinter.h>
+
+void bank2_func(int x) __banked
+{
+    (void)x;
+    puts("BANK2: hello from the second bank (W1)!");
+    puts("BANK2: window 1 phys page = ");
+    hex8(_io_page_w1);     /* should be BANK2's phys page */
+    putchar('\n');
+    putchar('2');
+    putchar('=');
+    putchar('0' + (x / 100) % 10);
+    putchar('0' + (x / 10) % 10);
+    putchar('0' + x % 10);
+    putchar('\n');
+}