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/errno/Makefile

@@ -0,0 +1,5 @@
+# Build errno_test.exe — uses lib/sprinter.lib in TINY memory mode.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := errno
+include $(PROJ_ROOT)/examples/example.mk

examples/errno/errno.c

@@ -0,0 +1,50 @@
+#include <stdio.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+/*
+ * Exercises errno + strerror + perror by triggering each failure path.
+ * No file system mutations are required — every call here is expected
+ * to fail by design.
+ */
+
+int main(void)
+{
+    puts("errno test:");
+    puts("");
+
+    /* 1. Open a file that does not exist. */
+    errno = 0;
+    int fd = open("NOSUCH.TXT", O_RDONLY);
+    printf("  open(\"NOSUCH.TXT\"): fd=%d  errno=%d  %s\n",
+           fd, errno, strerror(errno));
+
+    /* 2. Close a bogus file descriptor. */
+    errno = 0;
+    int r = close(0x7F);
+    printf("  close(0x7F): ret=%d  errno=%d  %s\n", r, errno, strerror(errno));
+
+    /* 3. perror() — convenience wrapper. */
+    errno = 0;
+    fd = open("ANOTHER.MIS", O_RDONLY);
+    perror("  open(\"ANOTHER.MIS\")");
+
+    /* 4. Delete something that isn't there. */
+    errno = 0;
+    r = unlink("PHANTOM.TMP");
+    perror("  unlink(\"PHANTOM.TMP\")");
+
+    /* 5. A few raw codes for the table. */
+    puts("");
+    puts("  raw codes:");
+    for (int i = 0; i <= 5; i++) {
+        printf("    %2d → %s\n", i, strerror(i));
+    }
+    printf("    %2d → %s  (out of range)\n", 99, strerror(99));
+
+    puts("");
+    puts("Press any key to exit.");
+    (void)getchar();
+    return 0;
+}