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/time/sleep.c

@@ -0,0 +1,30 @@
+/*
+ * sleep — block for N seconds using the 50 Hz frame interrupt.
+ *
+ * Sprinter ISR fires 50 times per second.  `halt` parks the CPU until
+ * the next IRQ, so 50 halts = ~1 second of wall clock.  This is the
+ * same trick solid-c uses in IO.ASM:285.
+ *
+ * Note: requires interrupts to be enabled (they are by default — ESTEX
+ * sets up IM 1 with the frame ISR before our program runs).
+ */
+
+#include <unistd.h>
+
+void sleep(unsigned int seconds) __naked
+{
+    (void)seconds;
+    __asm
+    inter:
+        ;; HL = seconds on entry (SDCC single int arg).
+        ld    a, h
+        or    a, l
+        ret   Z                     ; sleep(0) — return immediately
+        ld    b, #50                ; 50 halts per second (50 Hz interrupt)
+    inner:
+        halt
+        djnz  inner
+        dec   hl
+        jr    inter
+    __endasm;
+}