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/SPRN2K16.TXT

@@ -0,0 +1,34 @@
+*** Area: ZX.SPECTRUM                             Date:  4 Apr 01  6:41:06
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Eugene Ostroukhov
+*** Subj: Sprinter News
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Eugene!
+
+  <Saturday March 31 2001> <11:19> Eugene Ostroukhov wr®te to Ivan Mak:
+
+ >> �p¨ ¯p®£p ¬¬¨p®¢ ­¨¨ ­¨ç¥¬ ­¥ ®â«¨ç ¥âáï ®â Z80. „®¯®«­¨â¥«ì­ë¥
+ >> ãáâp®©á⢠ ¤®áâã¯­ë ¯® ª®¬ ­¤ ¬ IN ¨ OUT. ‚ ‘¯p¨­â¥p¥ ¯p®æ¥áá®p
+ >> ­¥¬­®£® p §®£­ ­ á 16 (¯® ¯ á¯®pâã) ¤® 21MHz.
+ EO> ƒ à ­â¨ï áâ ¡¨«ì­®áâ¨?
+
+�®«¥¥ ¤¢ãå ¤¥áï⪮¢ ¢®¯p®á®¢ á í⨬ ­¥ ¯®ï¢«ï«®áì. € ¢ á ¬®¬ ­ ç «¥
+¯p®¢®¤¨«¨áì ¨áá«¥¤®¢ ­¨ï ­  íâ®â áç¥â. „® 21MHz £®­¨âáï ¤ ¦¥ ¬®¤¨ä¨ª æ¨ï
+Z84C15 ­  10MHz.
+’ ª çâ® ¤«ï 16-£® § ¯ á ¥áâì. € ¢ëè¥ ­¥ áâ ¢¨¬ ¢ ¢¨¤ã ®âáãâá⢨ï â ª®¢®£®.
+16-ï £p ¤ æ¨ï ¤«ï Z84C14 ¬ ªá¨¬ «ì­ ï.
+
+ >> 800x600 ­¨ª ª®© TV ­¥ ¤ áâ, ­¥ ¡®«¥¥ 800x256 (280?)
+ >> �®«ìè¥ «¨­¨© ⮫쪮 ¨­â¥p«¥©á.
+ EO> 800x600 - áâ ­¤ àâ­®¥ à §à¥è¥­¨¥ ⥫¥¢¨§®à  ã ¯¨á¢.
+
+‚ ¨­â¥p«¥©á¥ ¨ á ®¡p¥§ ­­ë¬¨ ¢¥på­¨¬¨/­¨¦­¨¬¨ áâp®ª ¬¨ ­  90% TV... §­ ¥¬...
+
+        �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)