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;
+}