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/errno.c

@@ -0,0 +1,77 @@
+/*
+ * errno.c — strerror / perror over the SDCC-provided `errno` global.
+ *
+ * The message table mirrors the one in solid-c's IO.ASM (we kept the
+ * English wording for grep-ability).  ESTEX returns codes 0..32 in the
+ * meaningful range; anything beyond gets "Unknown error".
+ *
+ * Note: we deliberately do NOT define `_errno` here — SDCC's
+ * z80.lib/errno.rel provides it (a single int in _DATA), and our libc
+ * wrappers (read.c, open.c, etc.) just assign to `errno`.  This
+ * removes the "multiple definition of _errno" link warning.
+ */
+
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+/*
+ * Stored verbatim — pointers in the lookup table cost 2 bytes each plus
+ * the message bytes themselves.  Sentinel "" entries pad out gaps so
+ * indexing stays direct.
+ */
+static const char *const messages[] = {
+    /*  0 */ "No error",
+    /*  1 */ "Invalid function",
+    /*  2 */ "Invalid drive number",
+    /*  3 */ "File not found",
+    /*  4 */ "Path not found",
+    /*  5 */ "Invalid handle",
+    /*  6 */ "Too many open files",
+    /*  7 */ "File already exists",
+    /*  8 */ "File is read-only",
+    /*  9 */ "Root directory overflow",
+    /* 10 */ "No free space",
+    /* 11 */ "Directory not empty",
+    /* 12 */ "Can't delete current directory",
+    /* 13 */ "Invalid media",
+    /* 14 */ "Unknown operation",
+    /* 15 */ "Directory exists",
+    /* 16 */ "Invalid filename",
+    /* 17 */ "Invalid EXE file",
+    /* 18 */ "Not supported EXE file",
+    /* 19 */ "Access denied",
+    /* 20 */ "Device not ready",
+    /* 21 */ "Seek error",
+    /* 22 */ "Sector not found",
+    /* 23 */ "CRC error",
+    /* 24 */ "Write protect",
+    /* 25 */ "Read error",
+    /* 26 */ "Write error",
+    /* 27 */ "Drive failure",
+    /* 28 */ "RESERVED",
+    /* 29 */ "RESERVED",
+    /* 30 */ "Out of memory",
+    /* 31 */ "Invalid memory block",
+    /* 32 */ "Unknown error",
+};
+
+const int ESTEX_MAX_ERR = sizeof(messages) / sizeof(messages[0]) - 1;
+
+const char *strerror(int err)
+{
+    if (err < 0 || err > ESTEX_MAX_ERR) {
+        err = EUNKERR;
+    }
+    return messages[err];
+}
+
+void perror(const char *prefix)
+{
+    if (prefix && *prefix) {
+        fputs(prefix, stderr);
+        fputs(": ", stderr);
+    }
+    fputs(strerror(errno), stderr);
+    fputs("\r\n", stderr);
+}