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/solid_compat.c

@@ -0,0 +1,29 @@
+/*
+ * solid_compat.c — Solid-C compatibility helpers that need real code
+ * (rather than just header macros).
+ */
+
+#include <sprinter_compat.h>
+#include <ctype.h>
+
+char *strlwr(char *s)
+{
+    char *p = s;
+    while (*p) {
+        if (*p >= 'A' && *p <= 'Z') *p += 'a' - 'A';
+        p++;
+    }
+    return s;
+}
+
+char *strupr(char *s)
+{
+    char *p = s;
+    while (*p) {
+        if (*p >= 'a' && *p <= 'z') *p -= 'a' - 'A';
+        p++;
+    }
+    return s;
+}
+
+/* div() comes from SDCC's z80.lib. */