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/sprn2k64.txt

@@ -0,0 +1,35 @@
+*** Area: ZX.SPECTRUM                             Date: 21 Jan 03 16:37:47
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Maxim Timonin
+*** Subj: Sprinter
+
+        �ਢ¥âáâ¢ãî ‚ á, Maxim!
+
+  <Monday January 20 2003> <21:46> Maxim Timonin wr®te to Ivan Mak:
+
+ MT> ‚®¯à®á ç ©­¨ª  ¢ í⮬ ¤¥«¥:   ­  ALTER¥ ¬®¦­® á ¬ Z80 áí¬ã«¨à®¢ âì, ¢
+ MT> ¤®¯®«­¥­¨¨ ª ¢¨¤îª¥, Ay ¨ ¯à®ç¨¬?
+
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+᫨誮¬ ¬­®£® ¬¥áâ  (­¥ p áç¨â ­ ¡ë« Z80 çâ® ¡ë ¥£® ¢  «ìâ¥pã ¯¨å âì).
+
+ MT> € ¥á«¨ ¬®¦­®, â® ­¥ á⮨«® ¡ë ¯®¤ã¬ âì ¨ ® ⮬, ç⮡ë áí¬ã«¨à®¢ âì ­¥
+ MT> ¯à®áâ® Z80,   ¡®«¥¥ ¯à®¤¢¨­ãâë© ¨ ¡®«¥¥ "à §à來ë©" Z80-ᮢ¬¥á⨬ë©
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+
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+§ ­¨¬ îâ ®­¨ ¬¥áâ  ­  ¬­®£® ¬¥­ìè¥ ç¥¬ Z80 ¢ ᨫ㠯p¨¢ï§ª¨ ª  på¨â¥ªâãp¥ �‹Œ.
+‚ ç áâ­®á⨠32-塨â­ë© ¯p®æ¥áá®p á ç áâ®â®© 14MHz, ¨ 1 ⠪⠭  ª®¬ ­¤ã
+§ ­¨¬ ¥â ¯p¨¬¥p­® 1/3 ®â �‹Œ, çâ® ¢ ‘¯p¨­â¥p¥ á⮨â... (¯®p浪  600 ï祥ª). €
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+
+
+
+        �à®â®áë á¡¥¦ «¨. ‡¥à£¨ § ª®¯ «¨áì. �®à  ¨ ¬­¥ § ªà㣫ïâìáï. Ivan.
+
+ - � §¢®¤¨áì á奬ª , ¡®«ìè  ¨ ¬ «¥­ìª ... [Sprinter] Sprinter-II [Forth-CPU]
+
+... ivan_mak@mail.ru  *  http://winglion.spb.ru  *  http://www.petersplus.ru
+--- GoldED+/386 1.0.0
+ * Origin: The Sprinter has You! (2:5030/529.24)