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/SPRN2K49.TXT

@@ -0,0 +1,44 @@
+*** Area: ZX.SPECTRUM                             Date: 30 Apr 01 15:43:08
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Alexey Nikishin
+*** Subj: Sprinter techs
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Alexey!
+
+  <Thursday April 26 2001> <21:38> Alexey Nikishin wr®te to Ivan Mak:
+
+ AN> 1.‚aèa ª®­â®àa ¨á室­¨ª¨ ¯à®è¨¢ª¨ ­a í¬ã«ïæ¨î ᯥªâà㬮¢áª®£® ¦¥«¥§a
+ AN>   àaáªàë¢a¥â ¨«¨ ­¥â ?
+
+ˆá室­¨ª¨ ¯p®è¨¢ª¨ �‹Œ ¢ ¤ ­­ë© ¬®¬¥­â ­¥ p áªp뢠îâáï.
+H® ¯¥p¥£®¢®pë ¯® í⮬㠢®¯p®áã ¢®§¬®¦­ë...
+
+ AN> …᫨ ­¥â, â® ­ã¦­® ª®¢ëàïâáï ¢ ®âª®¬¯¨«¥­­®© ¯à®è¨¢ª¥ ¨«¨ áa¬®¬ã
+ AN> ¯¨áaâì á ­ã«ï ?
+
+Š®¢ëpïâìáï ¢ ®âª®¬¯¨«¥­®© ¯p®è¨¢ª¥ (�‹Œ-¢áª®©) ¡¥á¯®«¥§­®. ’ ¬ ¨­ä®p¬ æ¨ï ¤ ¦¥
+­ ¬ ­¥¨¢¥áâ­® ª ª p á¯®« £ ¥âáï, íâ® ª®¬¥pç¥áª ï â ©­  ä¨p¬ë ALTERA.
+
+ AN> Api ¤«ï ¨§¬¥­¥­¨ï ¯à®è¨¢ª¨ ¥áâì ?
+
+„«ï ­ ¯¨á ­¨ï ᢮¥© ¯p®è¨¢ª¨ ¥áâì ¯p®£p ¬¬  MAX-Plus II.
+…᫨ p §¡¨p ¥èìáï ¢ ¦¥«¥§¥, ­ ¯¨á âì ¯p®è¨¢ªã ¤«ï ¯p®á⮣®
+‘¯¥ªâp㬠 - ¤¥«® ®¤­®£® ¤­ï.
+
+€ ¢ ‘¯p¨­â¥p®¢áª®© ¯p®è¨¢ª¥ ¥áâì ­¥ª®â®pë¥ ¢¥é¨ (­®ã-å ã),
+ª®â®pë¥ ä¨p¬  �¥â¥pá ¥é¥ ­¥ £®â®¢  p áªp뢠âì.
+
+ AN> 2.‘奬a á¯à¨­â¥àa ¢ í«¥ªâà®­­®¬ ¢¨¤¥ ¯ã¡«¨æ¨àã¥âáï ?
+
+�®ª  ­¥â. H® ¥á«¨ ­ã¦­® ¤«ï ¤¥« , ®¯ïâì ¦¥, ¬®¦­® ¯¥p¥£®¢®p¨âì
+®¡ í⮬ ¢ ¬ë«¥ (­® íâ® á ­ ç «ìá⢮¬ ä¨p¬ë,   ­¥ á® ¬­®©).
+—¥p¥§ e-mail: peters@atlant.ru
+
+        �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)