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>

lib/Makefile

@@ -22,25 +22,44 @@ BUILD     := $(PROJ_ROOT)/lib/build
 
 # All libc C modules.
 LIBC_C := \
-    libc/io/atexit.c   libc/io/conio.c    libc/io/cprintf.c  libc/io/dir.c \
-    libc/io/videomode_raw.c \
-    libc/io/_errno_set.c \
-    libc/io/env.c      libc/io/errno.c    libc/io/fsdir.c \
-    libc/io/lseek.c    libc/io/mouse.c    libc/io/open.c \
-    libc/io/read.c     libc/io/sleep.c \
-    libc/io/time.c     libc/io/posix_time.c  libc/io/unlink.c \
+    libc/sys/atexit.c \
+    libc/conio/conio.c \
+    libc/conio/cprintf.c \
+    libc/io/dir.c \
+    libc/video/videomode_raw.c \
+    libc/errno/_errno_set.c \
+    libc/env/env.c \
+    libc/errno/errno.c \
+    libc/io/fsdir.c \
+    libc/io/lseek.c \
+    libc/mouse/mouse.c \
+    libc/io/open.c \
+    libc/io/read.c \
+    libc/time/sleep.c \
+    libc/time/time.c \
+    libc/time/posix_time.c \
+    libc/io/unlink.c \
     libc/io/stat.c \
-    libc/mem/bank_io_w3.c libc/mem/bank_io_w1.c libc/mem/mem_alloc.c \
-    libc/gfx/gfx_core.c       libc/gfx/gfx_raw_common.c \
-    libc/gfx/gfx_raw_256.c    libc/gfx/gfx_raw_16.c \
-    libc/gfx/gfx_256.c        libc/gfx/gfx_16.c \
-    libc/gfx/gfx_font.c       libc/gfx/gfx_text_256.c \
+    libc/mem/bank_io_w3.c \
+    libc/mem/bank_io_w1.c \
+    libc/mem/mem_alloc.c \
+    libc/gfx/gfx_core.c \
+    libc/gfx/gfx_raw_common.c \
+    libc/gfx/gfx_raw_256.c \
+    libc/gfx/gfx_raw_16.c \
+    libc/gfx/gfx_256.c \
+    libc/gfx/gfx_16.c \
+    libc/gfx/gfx_font.c \
+    libc/gfx/gfx_text_256.c \
     libc/gfx/gfx_text_16.c \
     libc/stdio/getchar.c \
-    libc/stdio/putchar.c libc/stdio/puts.c libc/stdio/file.c \
-    libc/stdio/hex_print.c libc/stdio/dec_print.c \
-    libc/stdio/solid_helpers.c \
-    libc/io/solid_compat.c
+    libc/stdio/putchar.c \
+    libc/stdio/puts.c \
+    libc/file/file.c \
+    libc/stdio/hex_print.c \
+    libc/stdio/dec_print.c \
+    libc/string/strlwr.c \
+    libc/string/strupr.c
 
 # Runtime modules to bundle (pulled by symbol references from libc-using code).
 # NOTE: runtime/bank.s is NOT bundled — its trampoline depends on the banking