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/assrtest/Makefile

@@ -0,0 +1,3 @@
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := assrtest
+include $(PROJ_ROOT)/app.mk

tests/assrtest/assrtest.c

@@ -0,0 +1,43 @@
+/*
+ * assrtest — exercise <assert.h> from SDCC's z80.lib.
+ *
+ * Tests both the disabled-debug (NDEBUG) form and the active form.
+ * The failing assert at the end prints an error message via printf
+ * (which routes through our putchar) and then halts.
+ */
+
+#include <stdio.h>
+#include <assert.h>
+
+static int divide(int a, int b)
+{
+    assert(b != 0);            /* must succeed for the test cases */
+    return a / b;
+}
+
+int main(int argc, char **argv)
+{
+    (void)argc; (void)argv;
+
+    puts("assrtest: exercising <assert.h>");
+
+    /* These pass. */
+    int q = divide(10, 2);
+    printf("  divide(10, 2) = %d\n", q);
+    assert(q == 5);
+
+    q = divide(-9, 3);
+    printf("  divide(-9, 3) = %d\n", q);
+    assert(q == -3);
+
+    puts("  all passing asserts OK");
+
+    /* Final, failing assert.  Prints "Assert(...) failed in function ..."
+     * then enters an infinite loop (SDCC __assert convention). */
+    puts("  triggering deliberate failing assert (system will hang)...");
+    int zero = 0;
+    (void)divide(1, zero);
+
+    /* unreachable */
+    return 0;
+}