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>

docs/reference/SPRINT2K/SPRN2K52.TXT

@@ -0,0 +1,28 @@
+*** Area: ZX.SPECTRUM                             Date: 30 Apr 01 15:44:42
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : UUCP
+*** Subj: Re[4]: ‘¯p¨­â¥p ¨ â.¯.
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Konstantin!
+
+  <Monday April 30 2001> <03:04> Konstantin A. Yeliseyev" <kay@udmnet.ru>
+Reply-To: "Konstantin A. Yeliseyev wr®te to Andrey Roska:
+
+ AR>> € ª ª �‹Œ í¬ã«¨à®¢ âì? H ¢¥à­®¥, âà㤭®¢ â® ¡ã¤¥â :)
+ Kk> �â®â ¢®¯à®á ­ ¯àï¬ãî á¢ï§ ­ á äã­ªæ¨ï¬¨ �‹Œ.
+ Kk> ‘®¢á¥¬ ­¥ ®¡ï§ â¥«ì­® í¬ã«¨à®¢ âì ¬ âà¨æã ­  ­¨§ª®¬ ã஢­¥.
+
+�®¤ ª ¦¤ãî ­®¢ãî ¯p®è¨¢ªã - ­®¢ë© í¬ã«ïâ®p,   ¯p¨ ¯¥p¥£p㧪¥
+¯p®è¨¢®ª ¯¥p¥£p㧪  í¬ã«ïâ®p  :-))
+
+P.S. ”¨p¬¥­­ë© €«ìâ¥p®¢áª¨© MAX-Plus-II ᨬã«ïâ®p í¬ã«¨â á奬ã
+‘¯p¨­â¥p  ¢ �‹Œ: 10 p¥ «ì­ëå _¬¨ªp®á¥ªã­¤_ §  ¤¥áïâì ᥪ㭤 ­  P-II-266.
+
+        �à®â®áë á¡¥¦ «¨. ‡¥à£¨ § ª®¯ «¨áì. �®à  ¨ ¬­¥ § ªà㣫ïâìáï. Ivan.
+
+  - � §¢®¤¨áì á奬ª , ¡®«ìè  ¨ ¬ «¥­ìª ... [ Sprinter-II ] [Forth-CPU] [ZX]
+
+... ivan_mak@mail.ru * http://st-rektal.chat.ru * http://www.atlant.ru/peters
+--- GoldED+/386 1.0.0
+ * Origin: The Sprinter has You! (2:5030/529.24)