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>

Makefile

@@ -1,58 +1,41 @@
 # Sprinter C Compiler — top-level Makefile
 #
-#   make             build host tools, libc archive, and all examples
+#   make             build host tools, libc archive, all tests, all apps
 #   make tools       build only host tools (mkexe)
 #   make lib         build lib/sprinter.lib (libc archive used by sprinter-cc)
-#   make examples    build all examples
-#   make floppy      package every example + test files into mame/v306/IMG/mc.img
+#   make tests       build all libc feature tests under tests/
+#   make examples    build all real applications under examples/
+#   make floppy      package every .exe + test fixtures into mame/v306/IMG/mc.img
 #   make check       run mkexe unit tests
 #   make clean       remove all build artefacts
 #   make sdcc        download/extract vendored SDCC
 #
 # Most heavy lifting is delegated to sub-Makefiles.
 
-EXAMPLES := hello banked bankedbg strtest cat seek malloc mem_test argv errno rt_test openenv ls conio attrprob timedir mouse banklocl stdlib assrtest ptime stattest filetest gfx_demo gfx_d16 gfx_text gfx_mous
+# Small libc-feature tests (one program per .c-language feature or libc API).
+TESTS := hello banked bankedbg strtest cat seek malloc mem_test argv errno \
+         rt_test openenv ls conio attrprob timedir mouse banklocl stdlib \
+         assrtest ptime stattest filetest gfx_demo gfx_d16 gfx_text gfx_mous
+
+# Larger end-user applications under examples/.
+APPS  := mdview
 
 MAME_DIR    := mame/v306
 FLOPPY_IMG  := $(MAME_DIR)/IMG/mc.img
 MAKE_DISK   := $(MAME_DIR)/make_disk.py
 
-EXE_FILES := \
-    examples/hello/hello.exe       \
-    examples/banked/banked.exe     \
-    examples/bankedbg/bankedbg.exe \
-    examples/strtest/strtest.exe   \
-    examples/cat/cat.exe           \
-    examples/seek/seek.exe         \
-    examples/malloc/malloc.exe     \
-    examples/mem_test/mem_test.exe \
-    examples/argv/argv.exe         \
-    examples/errno/errno.exe       \
-    examples/rt_test/rt_test.exe   \
-    examples/openenv/openenv.exe   \
-    examples/ls/ls.exe             \
-    examples/conio/conio.exe       \
-    examples/attrprob/attrprob.exe \
-    examples/timedir/timedir.exe   \
-    examples/mouse/mouse.exe       \
-    examples/banklocl/banklocl.exe \
-    examples/stdlib/stdlib.exe     \
-    examples/assrtest/assrtest.exe \
-    examples/ptime/ptime.exe       \
-    examples/stattest/stattest.exe \
-    examples/filetest/filetest.exe \
-    examples/gfx_demo/gfx_demo.exe \
-    examples/gfx_d16/gfx_d16.exe   \
-    examples/gfx_text/gfx_text.exe \
-    examples/gfx_mous/gfx_mous.exe
+TEST_EXES := $(foreach t,$(TESTS),tests/$(t)/$(t).exe)
+APP_EXES  := $(foreach a,$(APPS),examples/$(a)/$(a).exe)
+ALL_EXES  := $(TEST_EXES) $(APP_EXES)
 
 DATA_FILES := \
-    examples/cat/test.txt          \
-    examples/seek/big.txt
+    tests/cat/test.txt          \
+    tests/seek/big.txt          \
+    examples/mdview/SAMPLE.MD
 
-.PHONY: all tools lib examples check clean sdcc floppy $(EXAMPLES)
+.PHONY: all tools lib tests examples check clean sdcc floppy $(TESTS) $(APPS)
 
-all: tools lib examples
+all: tools lib tests examples
 
 tools:
 	$(MAKE) -C toolchain/mkexe
@@ -63,18 +46,22 @@ lib:
 check: tools
 	$(MAKE) -C toolchain/mkexe check
 
-examples: $(EXAMPLES)
+tests:    $(TESTS)
+examples: $(APPS)
 
-$(EXAMPLES): tools lib
+$(TESTS): tools lib
+	$(MAKE) -C tests/$@
+
+$(APPS):  tools lib
 	$(MAKE) -C examples/$@
 
 # Generate big.txt if missing (gen_bigfile.py creates 100 KB marker file).
-examples/seek/big.txt:
-	cd examples/seek && python3 gen_bigfile.py big.txt 102400
+tests/seek/big.txt:
+	cd tests/seek && python3 gen_bigfile.py big.txt 102400
 
-# Re-pack the MAME floppy image with every built example + needed data files.
-floppy: examples examples/seek/big.txt
-	python3 $(MAKE_DISK) $(FLOPPY_IMG) $(EXE_FILES) $(DATA_FILES)
+# Re-pack the MAME floppy image with every built exe + needed data files.
+floppy: tests examples tests/seek/big.txt
+	python3 $(MAKE_DISK) $(FLOPPY_IMG) $(ALL_EXES) $(DATA_FILES)
 	@echo
 	@echo "Floppy ready: $(FLOPPY_IMG)"
 	@echo "Run:  cd $(MAME_DIR) && ./run_mame.sh"
@@ -82,7 +69,8 @@ floppy: examples examples/seek/big.txt
 clean:
 	$(MAKE) -C toolchain/mkexe clean
 	$(MAKE) -C lib clean
-	@for e in $(EXAMPLES); do $(MAKE) -C examples/$$e clean; done
+	@for t in $(TESTS); do $(MAKE) -C tests/$$t clean; done
+	@for a in $(APPS);  do $(MAKE) -C examples/$$a clean; done
 
 sdcc:
 	bash third_party/setup-sdcc.sh