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/malloc/Makefile

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

tests/malloc/malloc.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/*
+ * Exercises SDCC's malloc / free.  The library pulls in heap.rel + malloc.rel
+ * from z80.lib automatically and inits the heap from crt0's _GSINIT chain.
+ * Default heap size is 1023 bytes — see docs/TODO.md for plans to grow it.
+ */
+
+int main(void)
+{
+    puts("malloc test:");
+
+    /* small allocations */
+    char *a = (char *)malloc(20);
+    char *b = (char *)malloc(40);
+    char *c = (char *)malloc(60);
+
+    printf("  a = %p\n", a);
+    printf("  b = %p\n", b);
+    printf("  c = %p\n", c);
+
+    if (a && b && c) {
+        strcpy(a, "first");
+        strcpy(b, "second-medium-length");
+        strcpy(c, "third-and-the-longest-of-them-all");
+        printf("  a: \"%s\"\n", a);
+        printf("  b: \"%s\"\n", b);
+        printf("  c: \"%s\"\n", c);
+    } else {
+        puts("  one of the allocations FAILED");
+    }
+
+    /* free b, alloc smaller — should fit in the b-shaped hole */
+    free(b);
+    char *d = (char *)malloc(15);
+    printf("  after free(b), d = %p (expected near b)\n", d);
+    if (d) {
+        strcpy(d, "reused");
+        printf("  d: \"%s\"\n", d);
+    }
+
+    /* drain a larger allocation to see when malloc actually says no */
+    int total = 0;
+    int count = 0;
+    for (;;) {
+        void *p = malloc(64);
+        if (!p) break;
+        total += 64;
+        count++;
+    }
+    printf("  alloc'd %d x 64-byte blocks before failure (total %d bytes)\n",
+           count, total);
+
+    free(a);
+    free(c);
+    free(d);
+    puts("");
+    puts("Press any key to exit.");
+    (void)getchar();
+    return 0;
+}