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/SPRN2K04.TXT

@@ -0,0 +1,32 @@
+*** Area: ZX.SPECTRUM                             Date: 18 Mar 01 11:55:28
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Maxim Vialkov
+*** Subj: ˆ­ä 
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Maxim!
+
+  <Thursday March 15 2001> <07:18> Maxim Vialkov wr®te to Kirill Frolov:
+
+ KF>> 1. ‘¯à¨­â¥à. $100 ⮫쪮 ¬ â쯫 â . www.peters.spb.ru ¨«¨ ª ª-â®
+
+ www.atlant.ru/peters ¨«¨ petersplus.da.ru
+
+ KF>> 2. Scorpion.   >= $30              www.scorpion.... ⮦¥ ª ª-â® â ª.
+ KF>> 3. Kay.        >= $30   H¥¬® á¥â¥© ­¥ ¯à¨§­ñâ. ’¥«¥ä®­ ⥡¥ ᪠§ «¨.
+ MV> �®ç¥¬ã â ª®© ᨫì­ë© à §¡à®á 業? ˆ«¨ "‘¯à¨­â¥à"- ªàãâ ­¥¬¥àï­®?
+
+ˆ¬¥­­® â ª. …᫨ 2 ¨ 3 ¤®¯®«­ïâì ª®­âp®««¥p ¬¨ �–-ª« ¢ë/¬ ãá , •„„, ‘Œ“Š-®¬, ¨
+ƒŒ•-®¬, ¯®«ãç¨âáï ¤®p®¦¥,   ¤® ‘¯p¨­â¥p  ¤ ¦¥ ­¥ ¤®âï­ãâ ¯® ᯮᮡ­®áâï¬,
+¨¡® ¢ ‘¯p¨­â¥p¥ ¢á¥ 㦥 ¢ ¬ â-¯« â¥ ¨¬¥¥âáï,
+
+ˆ 業  ‘¯p¨­â¥p  ¢ ᯨá®çª¥ ­¥¬­®£® § ¢ë襭 ... $95 ã ­¥£® :-)
+
+
+        �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)