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/video/videomode_raw.c

@@ -0,0 +1,46 @@
+/*
+ * videomode_raw.c — low-level ESTEX SETVMOD / GETVMOD ($50 / $51).
+ *
+ * Plain getters/setters with NO mode-class validation.  Used by both
+ * conio (text-validated public API) and gfx (graphics modes).  Lives
+ * in its own .c so a pure graphics program does not pull in the entire
+ * conio module to switch modes.
+ *
+ * Public conio functions in conio.c wrap these with a text-mode check;
+ * gfx_init / gfx_done in gfx_core.c call them directly.
+ */
+
+#include <stdint.h>
+#include <errno.h>
+
+uint8_t _videomode_raw_get(void) __naked
+{
+    __asm
+        push  ix
+        ld    c, #0x51              ; ESTEX GETVMOD
+        rst   #0x10
+        pop   ix
+        ;; uint8_t returns in A — ESTEX already put mode there.
+        ret
+    __endasm;
+}
+
+int _videomode_raw_set(uint8_t mode) __naked
+{
+    (void)mode;
+    __asm
+        ;; SDCC __sdcccall(1) passes uint8_t in A — leave it there.
+        push  ix
+        ld    bc, #0x0050           ; ESTEX SETVMOD (B=0 (page), C=0x50)
+        rst   #0x10
+        jr    c, _vmr_err
+        ld    de, #0
+        pop   ix
+        ret
+    _vmr_err:
+        call  __errno_set
+        ld    de, #-1
+        pop   ix
+        ret
+    __endasm;
+}