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/rt_test/Makefile

@@ -0,0 +1,5 @@
+# Build rt_test.exe — uses lib/sprinter.lib in TINY memory mode.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := rt_test
+include $(PROJ_ROOT)/app.mk

tests/rt_test/rt_test.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <setjmp.h>
+#include <sprinter_exit.h>
+
+static void bye1(void) { puts("[atexit] bye1 (registered first, runs last)"); }
+static void bye2(void) { puts("[atexit] bye2 (registered second)"); }
+static void bye3(void) { puts("[atexit] bye3 (registered third, runs first)"); }
+
+static jmp_buf env;
+static int     attempt;
+
+static void might_fail(int n)
+{
+    printf("  might_fail(%d) running...\n", n);
+    if (n == 2) {
+        puts("  -> longjmp(env, 42)");
+        longjmp(env, 42);
+        /* not reached */
+    }
+    printf("  might_fail(%d) returned normally\n", n);
+}
+
+int main(void)
+{
+    puts("Sprinter runtime features test:");
+    puts("");
+
+    /* --- atexit registration --- */
+    atexit(bye1);
+    atexit(bye2);
+    atexit(bye3);
+
+    /* --- setjmp / longjmp --- */
+    puts("setjmp/longjmp:");
+    int r = setjmp(env);
+    if (r == 0) {
+        attempt = 1;
+        puts("  first arrival from setjmp (r=0)");
+        might_fail(attempt);
+        attempt = 2;
+        might_fail(attempt);
+        puts("  -- this line is NOT reached");
+    } else {
+        printf("  resumed via longjmp, setjmp returned %d (attempt=%d)\n",
+               r, attempt);
+    }
+
+    /* --- sleep --- */
+    puts("");
+    puts("sleep(5): pausing 5 seconds...");
+    sleep(5);
+    puts("  done.");
+
+    puts("");
+    puts("Press any key, atexit chain will run after.");
+    (void)getchar();
+
+    /* Use exit() to drive the atexit chain (vs returning from main, which
+     * doesn't necessarily go through exit() under SDCC). */
+    exit(0);
+    return 0;
+}