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/stdio/puts.c

@@ -14,17 +14,11 @@
  *   - Avoid trailing PUTCHAR after PCHARS — empirically that sometimes
  *     drops the next char.  Embed the line ending inside the PCHARS
  *     buffer instead.
- *   - Strings longer than the buffer fall back to per-char putchar so
- *     we never silently truncate.
  */
 
 #include <stdio.h>
 #include <stdint.h>
 
-#define PUTS_BUF_SIZE  256       /* body bytes before CR expansion */
-
-static char puts_buf[PUTS_BUF_SIZE + 3];   /* +3 for trailing CR LF NUL */
-
 static void pchars(const char *s) __naked
 {
     (void)s;
@@ -37,33 +31,26 @@ static void pchars(const char *s) __naked
     __endasm;
 }
 
-int puts(const char *s)
+char puts(const char *s) __naked
 {
-    uint16_t n = 0;
-    uint16_t i = 0;
-
-    while (s[i] && n < PUTS_BUF_SIZE - 1) {
-        char c = s[i++];
-        if (c == '\n') {
-            puts_buf[n++] = '\r';
-            puts_buf[n++] = '\n';
-        } else {
-            puts_buf[n++] = c;
-        }
-    }
-
-    if (s[i]) {
-        /* Overflow — char-by-char fallback so we never truncate. */
-        for (uint16_t k = 0; s[k]; k++)
-            putchar((unsigned char)s[k]);
-        putchar('\n');
-        return 0;
-    }
-
-    puts_buf[n++] = '\r';
-    puts_buf[n++] = '\n';
-    puts_buf[n]   = 0;
-
-    pchars(puts_buf);
-    return 0;
+    (void)s;    
+    __asm
+    puts_:
+        ld    a, (hl)
+        or    a
+        jr    z, fin_
+        push  hl
+        ld    l, a
+        ld    h, #0
+        call  _putchar
+        pop   hl
+        inc   hl
+        jp    puts_
+        ;
+    fin_:	
+        ld    l, #0x0A
+        ld    h, #0
+        call  _putchar
+        ret
+    __endasm;
 }