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/banked/Makefile

@@ -0,0 +1,11 @@
+# Build banked.exe — HUGE memory mode (small + banked code in W3).
+#
+# Layout:
+#   CODE/HOME at 0x4100 (W1), DATA at 0x8000 (W2), banks at 0x{N}C000 (W3).
+#   crt0_banked.s loads N banks from the .EXE before calling main.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := banked
+MEMORY    := huge
+EXTRA_FLAGS := --bank 1=bank1.c --bank 2=bank2.c
+include $(PROJ_ROOT)/app.mk

tests/banked/bank1.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <sprinter.h>
+
+void bank1_func(int x) __banked
+{
+    (void)x;
+    puts("BANK1: hello from a banked function!");
+    puts("BANK1: window 3 phys page = ");
+    hex8(_io_page_w3);     /* should be BANK1's phys page */
+    putchar('\n');
+    putchar('1');
+    putchar('=');
+    putchar('0' + (x / 10) % 10);
+    putchar('0' + x % 10);
+    putchar('\n');
+}

tests/banked/bank2.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <sprinter.h>
+
+void bank2_func(int x) __banked
+{
+    (void)x;
+    puts("BANK2: hello from the second bank!");
+    puts("BANK2: window 3 phys page = ");
+    hex8(_io_page_w3);     /* should be BANK2's phys page */
+    putchar('\n');
+    putchar('2');
+    putchar('=');
+    putchar('0' + (x / 100) % 10);
+    putchar('0' + (x / 10) % 10);
+    putchar('0' + x % 10);
+    putchar('\n');
+}

tests/banked/banked.c

@@ -0,0 +1,47 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <sprinter.h>
+
+/* crt0_banked.s reads this constant to know how many banks to load. */
+const uint8_t n_banks = 2;
+
+void bank1_func(int x) __banked;
+void bank2_func(int x) __banked;
+
+extern uint8_t bank_pages[];   /* filled by crt0_banked.s */
+
+static void show_w3(const char *who)
+{
+    puts(who);
+    hex8(_io_page_w3);     /* current physical page mapped in window 3 */
+    putchar('\n');
+}
+
+int main(void)
+{
+    puts("HOME: program start.");
+#ifdef DEBUG_RT
+    printf("HOME: w2_self_allocated = %u\n", w2_self_allocated);
+#endif
+    show_w3("HOME: window 3 phys page = ");
+
+    puts("HOME: bank_pages[1] (BANK1 phys) = ");
+    hex8(bank_pages[1]);
+    putchar('\n');
+    puts("HOME: bank_pages[2] (BANK2 phys) = ");
+    hex8(bank_pages[2]);
+    putchar('\n');
+
+    puts("HOME: calling bank1_func(42)...");
+    bank1_func(42);
+    show_w3("HOME: back, window 3 phys = ");
+
+    puts("HOME: calling bank2_func(123)...");
+    bank2_func(123);
+    show_w3("HOME: back, window 3 phys = ");
+
+    puts("HOME: done.");
+    puts("Press any key to exit...");
+    (void)getchar();
+    return 0;
+}