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/include/unistd.h

@@ -0,0 +1,39 @@
+/*
+ * unistd.h — POSIX-style file descriptor I/O for Sprinter ESTEX.
+ *
+ * Each call maps to a single ESTEX RST 10h function:
+ *   read   → $13  write  → $14
+ *   close  → $12  unlink → $0E
+ *
+ * On error every call returns -1 (the ESTEX error code is not exposed yet;
+ * a future errno mechanism will surface it).
+ */
+
+#ifndef UNISTD_H
+#define UNISTD_H
+
+#include <stddef.h>      /* size_t */
+
+/* lseek whence values (POSIX). */
+#define SEEK_SET  0
+#define SEEK_CUR  1
+#define SEEK_END  2
+
+int   read (int fd, void *buf, size_t n);
+int   write(int fd, const void *buf, size_t n);
+int   close(int fd);
+int   unlink(const char *path);
+long  lseek(int fd, long offset, int whence);
+
+/* Block the calling task for `seconds` seconds (50 Hz IRQ-based timer). */
+void  sleep(unsigned int seconds);
+
+/* Directory operations (ESTEX $1B-$1E).  All return 0 on success and -1
+ * with errno set on failure; getcwd returns the buffer on success or NULL.
+ * size is ignored — ESTEX always wants a 256-byte buffer. */
+int   mkdir (const char *path);
+int   rmdir (const char *path);
+int   chdir (const char *path);
+char *getcwd(char *buf, size_t size);
+
+#endif