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/conio2/Makefile

@@ -0,0 +1,5 @@
+# Build conio_test.exe — uses lib/sprinter.lib in TINY memory mode.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := conio2
+include $(PROJ_ROOT)/app.mk

tests/conio2/conio2.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#include <conio.h>
+#include <unistd.h>     /* sleep */
+
+/*
+ * Exercises every conio function:
+ *   clrscr  — clear the screen
+ *   gotoxy  — move cursor to (x, y), 0-based
+ *   putch   — single character
+ *   cputs   — string without auto-newline
+ *   kbhit   — non-blocking key probe
+ *   getch   — blocking, no echo
+ *   getche  — blocking, with echo
+ *
+ * Sprinter native text mode is 80×32, so we anchor corners at
+ * (0,0), (79,0), (0,31), (79,31).
+ */
+
+int main(void)
+{
+    /* 1. clrscr + diagnostic probes + a centered banner.
+     *
+     * Row-0 probe: figure out why [NW]/[NE] don't paint.
+     *   '0','1','2','3' — single putch at row 0, cols 0/5/40/79
+     *   "R0"            — cputs at row 0, col 10
+     *   'a','b','c','R1'— same pattern on row 1 (control)
+     */
+    clrscr();
+
+    gotoxy(0, 0);  putch('0');
+    textattr(0x70);
+    gotoxy(1, 0);  putch('1');
+
+    set_putch_raw_mode(1);
+
+    textattr(0x0F);
+    gotoxy(4, 3);
+    for (int i = 0; i < 16; i++) {
+        putch(i < 10 ? '0' + i : 'A' - 10 + i);
+        putch(' ');
+    }
+
+    for (int j = 0; j < 16; j++) {
+        gotoxy(2, 4 + j);
+        textattr(0x0F);
+        putch(j < 10 ? '0' + j : 'A' - 10 + j);
+        putch(' ');
+        for (int i = 0; i < 16; i++) {
+            textattr((j << 4) + i);
+            putch((j << 4) + i); putch(' ');
+        }
+    }
+
+    set_putch_raw_mode(0);
+    gotoxy(0,22);
+    set_text_attr(0x000E);
+//    cputs("Test message line");
+    cputs("Test message line 1\nTest message line2\rline3\n\r\n\r1234\t5678\b90");
+
+    (void)getch();
+
+    return 0;
+}