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/fcntl.h

@@ -0,0 +1,31 @@
+/*
+ * fcntl.h — open / creat for Sprinter ESTEX.
+ *
+ * POSIX-style flag bits.  The low two bits select access mode (matches
+ * POSIX numbering — RDONLY=0, WRONLY=1, RDWR=2 — and is translated to
+ * the ESTEX OPEN $11 convention inside open()).
+ *
+ * The other flags map onto ESTEX calls like this:
+ *   O_CREAT  + O_EXCL   → $0B (CREATE_NEW, fails if exists)
+ *   O_CREAT  + O_TRUNC  → $0A (CREATE, truncates existing)
+ *   O_CREAT  alone      → try $11 (OPEN); on ENOENT fall back to $0A
+ *   no O_CREAT          → $11 (OPEN, fails if missing)
+ *   O_APPEND            → after open, $15 lseek(0, SEEK_END)
+ */
+
+#ifndef FCNTL_H
+#define FCNTL_H
+
+#define O_RDONLY  0
+#define O_WRONLY  1
+#define O_RDWR    2
+
+#define O_CREAT   0x040
+#define O_EXCL    0x080
+#define O_TRUNC   0x200
+#define O_APPEND  0x400
+
+int open (const char *path, int flags);
+int creat(const char *path, int mode);   /* mode arg ignored on Sprinter */
+
+#endif