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>

libc/io/atexit.c

@@ -0,0 +1,57 @@
+/*
+ * atexit + exit + _exit.
+ *
+ *   atexit(fn)  — register fn to be called at normal termination (max 8).
+ *   exit(code)  — run the atexit chain in LIFO, then ESTEX EXIT.
+ *   _exit(code) — POSIX raw exit: skip the chain, go straight to ESTEX.
+ *
+ * These functions own termination entirely; crt0.s only does inline RST
+ * 10h #41 when main returns without an explicit exit().  That path skips
+ * the atexit chain — programs that need handlers should call exit() at
+ * the bottom of main (or return through a wrapper).
+ */
+
+#include <stdlib.h>
+#include <sprinter_exit.h>
+
+#define ATEXIT_MAX  8
+
+static void (*atexit_stack[ATEXIT_MAX])(void);
+static int   atexit_top = 0;
+
+int atexit(void (*fn)(void))
+{
+    if (atexit_top >= ATEXIT_MAX) {
+        return -1;
+    }
+    atexit_stack[atexit_top++] = fn;
+    return 0;
+}
+
+/* exit() — runs the chain, then performs the raw ESTEX EXIT. */
+void exit(int code)
+{
+    while (atexit_top > 0) {
+        void (*fn)(void) = atexit_stack[--atexit_top];
+        if (fn) {
+            fn();
+        }
+    }
+    _exit(code);   /* falls into the inline-asm raw exit below */
+}
+
+/* _exit() — POSIX raw termination, no atexit chain. */
+void _exit(int code) __naked
+{
+    (void)code;
+    __asm
+        ;; HL = code (single int arg).
+        ld    a, l
+        ld    b, a
+        ld    c, #0x41        ; ESTEX EXIT
+        rst   #0x10
+        ;; Should not return.
+1$:     halt
+        jr    1$
+    __endasm;
+}