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>

.gitignore

@@ -9,7 +9,8 @@ build/
 # sprinter-cc per-example intermediate directory
 .sprinter-cc-*/
 
-# Per-example final/intermediate outputs landing alongside the source
+# Per-program final/intermediate outputs landing alongside the source
+# (real apps under examples/ and libc feature tests under tests/).
 examples/*/*.exe
 examples/*/*.asm
 examples/*/*.lst
@@ -23,6 +24,19 @@ examples/*/*.cdb
 examples/*/*.mem
 examples/*/*.rst
 
+tests/*/*.exe
+tests/*/*.asm
+tests/*/*.lst
+tests/*/*.lk
+tests/*/*.ihx
+tests/*/*.noi
+tests/*/*.sym
+tests/*/*.map
+tests/*/*.rel
+tests/*/*.cdb
+tests/*/*.mem
+tests/*/*.rst
+
 # libc archive (built from libc/, see lib/Makefile)
 lib/*.lib