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>

tests/bankedbg/Makefile

@@ -0,0 +1,12 @@
+# Build banked_big.exe — BIG memory mode (tiny + banked code in W1).
+#
+# Layout:
+#   CODE/HOME at 0x8100 (W2), DATA chained after CODE (W2)
+#   Banks at virtual 0x{N}4000 — physical pages mapped into W1
+#   crt0_banked.s + bank.s built with BANK_W1=1 by sprinter-cc
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := bankedbg
+MEMORY    := big
+EXTRA_FLAGS := --bank 1=bank1.c --bank 2=bank2.c
+include $(PROJ_ROOT)/app.mk