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/ptime/Makefile

@@ -0,0 +1,3 @@
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := ptime
+include $(PROJ_ROOT)/app.mk

tests/ptime/ptime.c

@@ -0,0 +1,44 @@
+/*
+ * ptime — exercise POSIX <time.h>.  All wired through SDCC's z80.lib
+ * time.rel which calls our RtcRead() (bridged to ESTEX SYSTIME).
+ */
+
+#include <stdio.h>
+#include <time.h>
+
+int main(void)
+{
+    /* time() returns seconds since 1970-01-01 (Unix epoch). */
+    time_t now;
+    time(&now);
+    printf("time()      = %lu epoch seconds\n", now);
+
+    /* localtime() decomposes into struct tm. */
+    struct tm *lt = localtime(&now);
+    printf("localtime() = %04d-%02d-%02d %02d:%02d:%02d  (wday=%u)\n",
+           lt->tm_year + 1900,
+           lt->tm_mon  + 1,
+           lt->tm_mday,
+           lt->tm_hour,
+           lt->tm_min,
+           lt->tm_sec,
+           (unsigned)lt->tm_wday);
+
+    /* asctime() formats it as a 25-char string. */
+    printf("asctime()   = %s", asctime(lt));    /* asctime adds \n */
+
+    /* mktime() round-trip — should recover the same epoch. */
+    time_t round = mktime(lt);
+    printf("mktime()    = %lu  (round-trip %s original)\n",
+           round, (round == now ? "==" : "!="));
+
+    /* Native datetime_t API still works alongside. */
+    datetime_t dt;
+    getdatetime(&dt);
+    printf("native      = %04u-%02u-%02u %02u:%02u:%02u  (dow=%u)\n",
+           dt.year, dt.month, dt.day,
+           dt.hour, dt.minute, dt.second,
+           (unsigned)dt.dow);
+
+    return 0;
+}