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/putchar.c

@@ -19,23 +19,15 @@ int putchar(int c) __naked
 {
     (void)c;
     __asm
-        ld    a, l                       ; SDCC __sdcccall(1) int → HL
-        push  ix
+        ld    a, l
         cp    #0x0A
-        jr    nz, _pc_emit
-        ld    a, #0x0D                   ; CR before LF
-        push  af
+        jr    nz, cputc
+        call  cputc
+        ld    a, #0x0D
+cputc:
         ld    c, #0x5B
         rst   #0x10
-        pop   af
-        ld    a, #0x0A
-    _pc_emit:
-        push  af
-        ld    c, #0x5B
-        rst   #0x10
-        pop   af
-        pop   ix
-        ld    e, a
+        ld    e, l
         ld    d, #0
         ret
     __endasm;