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/stdlib/stdlib.c

@@ -0,0 +1,63 @@
+/*
+ * stdlib_test — verifies that SDCC's z80.lib stdlib functions (atoi,
+ * strtol, rand, qsort, bsearch, abs, ldiv) work in our environment.
+ * If they all behave correctly, we can save ourselves the labour of
+ * reimplementing them in libc/stdlib.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static int cmp_int(const void *a, const void *b)
+{
+    int ia = *(const int *)a;
+    int ib = *(const int *)b;
+    return (ia > ib) - (ia < ib);
+}
+
+int main(int argc, char **argv)
+{
+    /* atoi / strtol */
+    int       v_atoi  = atoi("12345");
+    long      v_strtol = strtol("-9876", NULL, 10);
+    long      v_hex    = strtol("ff", NULL, 16);
+    printf("atoi(\"12345\") = %d (expected 12345)\n", v_atoi);
+    printf("strtol(\"-9876\",,10) = %ld (expected -9876)\n", v_strtol);
+    printf("strtol(\"ff\",,16) = %ld (expected 255)\n", v_hex);
+
+    /* abs / ldiv */
+    printf("abs(-7) = %d (expected 7)\n", abs(-7));
+    {
+        ldiv_t q = ldiv(100L, 7L);
+        printf("ldiv(100,7) = {q=%ld, r=%ld} (expected {14, 2})\n",
+               q.quot, q.rem);
+    }
+
+    /* rand / srand — deterministic with fixed seed */
+    srand(42);
+    printf("rand x3 (seed 42): %d %d %d\n", rand(), rand(), rand());
+
+    /* qsort */
+    {
+        int arr[] = { 5, 1, 4, 2, 8, 3, 7, 6, 0, 9 };
+        qsort(arr, 10, sizeof(int), cmp_int);
+        printf("qsort:");
+        for (int i = 0; i < 10; i++) printf(" %d", arr[i]);
+        printf(" (expected 0..9 sorted)\n");
+
+        /* bsearch */
+        int key = 7;
+        int *p = (int *)bsearch(&key, arr, 10, sizeof(int), cmp_int);
+        printf("bsearch(7) -> %s (expected found)\n",
+               (p && *p == 7) ? "found" : "MISS");
+    }
+
+    /* argv-as-int: typical CLI use */
+    if (argc > 1) {
+        printf("argv[1] as int: %d\n", atoi(argv[1]));
+    }
+
+    puts("\nall SDCC stdlib functions reachable.");
+    return 0;
+}