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>

libc/errno/_errno_set.c

@@ -0,0 +1,40 @@
+/*
+ * _errno_set — set `errno` from an ESTEX error code (0..255 in A).
+ *
+ * Replaces the inline pattern
+ *      ld    (_errno),    a
+ *      xor   a, a
+ *      ld    (_errno+1),  a            ; 7 bytes per error path
+ * with a single
+ *      call  __errno_set                ; 3 bytes per error path
+ *
+ * Saves ~4 bytes at every libc error handler that converts an ESTEX
+ * code into the C-side `errno`.  Helper itself is 7 bytes; with 10+
+ * error paths in our libc the size win is net-positive.
+ *
+ * ABI:
+ *   in:    A  = ESTEX error code (0..255)
+ *   out:   HL = A (zero-extended); errno fully overwritten so a prior
+ *               large value (e.g. errno = -1) can't leak its high byte.
+ *   clobbers: HL, AF flags.  Caller must not depend on HL afterwards.
+ *
+ * Defensive 16-bit store — see chat 2026-06-02: if anyone ever assigns
+ * errno via C (`errno = -1`), the high byte becomes 0xFF, and a partial
+ * 8-bit write here would leave that 0xFF in place.  Always writing the
+ * full word keeps errno honest regardless of who set it last.
+ */
+
+void _errno_set(unsigned char code) __naked
+{
+    (void)code;
+    __asm
+        ;; __sdcccall(1): single uint8_t arg already in A.
+        ;; Write the two bytes separately so HL/BC/DE/IX/IY remain
+        ;; untouched.  Only A is clobbered: it is the input register,
+        ;; and ABI does not require preserving it across a void call.
+        ld    (_errno), a            ; low byte = code
+        xor   a, a
+        ld    (_errno+1), a          ; high byte = 0
+        ret
+    __endasm;
+}

libc/errno/errno.c

@@ -0,0 +1,77 @@
+/*
+ * errno.c — strerror / perror over the SDCC-provided `errno` global.
+ *
+ * The message table mirrors the one in solid-c's IO.ASM (we kept the
+ * English wording for grep-ability).  ESTEX returns codes 0..32 in the
+ * meaningful range; anything beyond gets "Unknown error".
+ *
+ * Note: we deliberately do NOT define `_errno` here — SDCC's
+ * z80.lib/errno.rel provides it (a single int in _DATA), and our libc
+ * wrappers (read.c, open.c, etc.) just assign to `errno`.  This
+ * removes the "multiple definition of _errno" link warning.
+ */
+
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+/*
+ * Stored verbatim — pointers in the lookup table cost 2 bytes each plus
+ * the message bytes themselves.  Sentinel "" entries pad out gaps so
+ * indexing stays direct.
+ */
+static const char *const messages[] = {
+    /*  0 */ "No error",
+    /*  1 */ "Invalid function",
+    /*  2 */ "Invalid drive number",
+    /*  3 */ "File not found",
+    /*  4 */ "Path not found",
+    /*  5 */ "Invalid handle",
+    /*  6 */ "Too many open files",
+    /*  7 */ "File already exists",
+    /*  8 */ "File is read-only",
+    /*  9 */ "Root directory overflow",
+    /* 10 */ "No free space",
+    /* 11 */ "Directory not empty",
+    /* 12 */ "Can't delete current directory",
+    /* 13 */ "Invalid media",
+    /* 14 */ "Unknown operation",
+    /* 15 */ "Directory exists",
+    /* 16 */ "Invalid filename",
+    /* 17 */ "Invalid EXE file",
+    /* 18 */ "Not supported EXE file",
+    /* 19 */ "Access denied",
+    /* 20 */ "Device not ready",
+    /* 21 */ "Seek error",
+    /* 22 */ "Sector not found",
+    /* 23 */ "CRC error",
+    /* 24 */ "Write protect",
+    /* 25 */ "Read error",
+    /* 26 */ "Write error",
+    /* 27 */ "Drive failure",
+    /* 28 */ "RESERVED",
+    /* 29 */ "RESERVED",
+    /* 30 */ "Out of memory",
+    /* 31 */ "Invalid memory block",
+    /* 32 */ "Unknown error",
+};
+
+const int ESTEX_MAX_ERR = sizeof(messages) / sizeof(messages[0]) - 1;
+
+const char *strerror(int err)
+{
+    if (err < 0 || err > ESTEX_MAX_ERR) {
+        err = EUNKERR;
+    }
+    return messages[err];
+}
+
+void perror(const char *prefix)
+{
+    if (prefix && *prefix) {
+        fputs(prefix, stderr);
+        fputs(": ", stderr);
+    }
+    fputs(strerror(errno), stderr);
+    fputs("\r\n", stderr);
+}