Add full compiler toolchain, libc, examples and reference docs

First substantive commit: the entire Sprinter C compiler tree on top of
the bare README+gitignore initial commit.

What's in here:
  bin/sprinter-cc        — driver script invoking SDCC + linker + mkexe
  libc/                  — Sprinter-specific libc layer over ESTEX/BIOS
                           (conio, gfx, io, mem, stdio + headers)
  runtime/               — crt0 variants (default/small/banked/minimal)
                           + heap + bank trampolines
  toolchain/             — mkexe (SprintEXE packer, C + tests)
  examples/              — 30 demo programs (gfx, file I/O, env, time, …)
  lib/Makefile           — builds the libc archive (sprinter.lib)
  docs/                  — converted Sprinter manuals + asm reference samples
  third_party/           — solid-c reference compiler dump + sdcc setup script
  release_docs/          — packaging / release notes

gitignore overhaul:
  • Drop dangerous blanket patterns: *.asm (would hide docs/samples/*.asm)
    and *.exe (case-insensitive match was hiding third_party/solid-c/*.EXE
    on macOS APFS).  Replaced with examples/*/*.{asm,exe,…} and lib/*.lib.
  • Restore tracking of toolchain/mkexe/tests/{one,big}.bin — those are
    INPUT fixtures, not build outputs.
  • Collapse the duplicated SDCC/C/Sdcc sections into one section per
    concern (build outputs / vendored / OS-junk).
  • Add .sprinter-cc-*/, build/ (catches lib/build/ too), .claude/.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

examples/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)/examples/example.mk

examples/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;
+}