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/openenv/openenv.c

@@ -0,0 +1,153 @@
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <sprinter.h>
+
+/*
+ * Exercises the new open() state machine + ESTEX env API.
+ *
+ * Touches the floppy: creates TMP1.TXT and TMP2.TXT, writes a few bytes,
+ * verifies O_CREAT / O_EXCL / O_TRUNC / O_APPEND behaviour, then deletes.
+ */
+ 
+extern uint8_t estex_file_handle;
+
+
+static void show_errno(const char *label)
+{
+    printf("  %s: errno=%d \"%s\"\n", label, errno, strerror(errno));
+}
+
+int main(void)
+{
+    puts("Sprinter open() + env API test");
+    puts("");
+
+    printf("estex_file_handle: fd=%u\n", estex_file_handle);
+
+    /* --- open() state machine ------------------------------------- */
+
+    /* 1. O_CREAT|O_EXCL: must succeed first time, must fail on retry. */
+    unlink("TMP1.TXT");          /* clean slate */
+    errno = 0;
+    int fd = open("TMP1.TXT", O_WRONLY | O_CREAT | O_EXCL);
+    printf("create-new TMP1.TXT (1st): fd=%d\n", fd);
+    show_errno("    after 1st");
+    if (fd >= 0) {
+        write(fd, "abcdef", 6);
+        close(fd);
+    }
+
+    errno = 0;
+    fd = open("TMP1.TXT", O_WRONLY | O_CREAT | O_EXCL);
+    printf("create-new TMP1.TXT (2nd, should fail with EEXIST): fd=%d\n", fd);
+    show_errno("    after 2nd");
+
+    /* 2. O_CREAT|O_TRUNC: file is reset. */
+    errno = 0;
+    fd = open("TMP1.TXT", O_WRONLY | O_CREAT | O_TRUNC);
+    printf("create-trunc TMP1.TXT: fd=%d\n", fd);
+    if (fd >= 0) {
+        write(fd, "XYZ", 3);
+        close(fd);
+    }
+
+    /* Read back to confirm truncation overwrote "abcdef". */
+    char buf[16];
+    fd = open("TMP1.TXT", O_RDONLY);
+    int n = read(fd, buf, sizeof(buf) - 1);
+    if (n < 0) n = 0;
+    buf[n] = 0;
+    close(fd);
+    printf("contents after trunc+write: \"%s\" (expect \"XYZ\")\n", buf);
+
+    /* 3. O_APPEND: writes go to end. */
+    fd = open("TMP1.TXT", O_WRONLY | O_APPEND);
+    if (fd >= 0) {
+        write(fd, "+APP", 4);
+        close(fd);
+    }
+    fd = open("TMP1.TXT", O_RDONLY);
+    n = read(fd, buf, sizeof(buf) - 1);
+    if (n < 0) n = 0;
+    buf[n] = 0;
+    close(fd);
+    printf("contents after append:      \"%s\" (expect \"XYZ+APP\")\n", buf);
+
+    /* 4. O_CREAT alone (no EXCL/TRUNC): existing file opens, new file is
+     *    created.  We open TMP1 (must succeed without truncating) then
+     *    TMP2 (must be created fresh). */
+    fd = open("TMP1.TXT", O_RDONLY | O_CREAT);
+    n = read(fd, buf, sizeof(buf) - 1);
+    if (n < 0) n = 0;
+    buf[n] = 0;
+    close(fd);
+    printf("O_CREAT on existing TMP1:    \"%s\" (expect \"XYZ+APP\")\n", buf);
+
+    unlink("TMP2.TXT");
+    fd = open("TMP2.TXT", O_WRONLY | O_CREAT);
+    printf("O_CREAT on missing TMP2:     fd=%d\n", fd);
+    if (fd >= 0) {
+        write(fd, "fresh", 5);
+        close(fd);
+    }
+
+    /* cleanup */
+    unlink("TMP1.TXT");
+    unlink("TMP2.TXT");
+
+    /* --- env API -------------------------------------------------- */
+    puts("");
+    puts("env API:");
+
+    errno = 0;
+    char *v = getenv("PATH");
+    printf("  getenv(\"PATH\") = %p", v);
+    if (v) {
+        printf(" -> \"%s\"", v);
+    } else {
+        printf(" (NULL, errno=%d)", errno);
+    }
+    putchar('\n');
+
+    errno = 0;
+    int rc = putenv("SPRINTER_HELLO=world");
+    printf("  putenv(\"SPRINTER_HELLO=world\"): rc=%d\n", rc);
+    if (rc != 0) show_errno("    after putenv");
+
+    errno = 0;
+    v = getenv("SPRINTER_HELLO");
+    printf("  getenv(\"SPRINTER_HELLO\") = %p", v);
+    if (v) {
+        printf(" -> \"%s\"", v);
+    } else {
+        printf(" (NULL, errno=%d)", errno);
+    }
+    putchar('\n');
+
+    /* --- sysenv: dump the whole environment block ----------------- */
+    puts("");
+    puts("sysenv:");
+    {
+        static char envbuf[512];
+        errno = 0;
+        char *r = sysenv(envbuf);
+        if (r == (char *)-1) {
+            show_errno("    sysenv failed");
+        } else {
+            printf("  sysenv(%p) returned %p\n", envbuf, r);
+            int n = 0;
+            for (char *p = r; *p; p += strlen(p) + 1) {
+                printf("    [%d] %s\n", n++, p);
+            }
+            printf("  total: %d variable(s)\n", n);
+        }
+    }
+
+    puts("");
+    puts("Press any key to exit.");
+    (void)getchar();
+    return 0;
+}