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/getchar.c

@@ -17,13 +17,7 @@ int getchar(void) __naked
         ld    c, #0x30        ; ESTEX WAITKEY
         rst   #0x10
         pop   ix
-        ld    a, e            ; E = ASCII (already the low byte of our return DE)
-        or    a, a
-        jr    Z, no_ascii
         ld    d, #0
         ret
-    no_ascii:
-        ld    de, #-1
-        ret
     __endasm;
 }