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/SPRN2K07.TXT

@@ -0,0 +1,29 @@
+*** Area: ZX.SPECTRUM                             Date: 22 Mar 01 11:31:25
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Stanislav Udin
+*** Subj: Ž ‘¯p¨­â¥p¥.
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Stanislav!
+
+  <Thursday March 22 2001> <11:33> Stanislav Udin wr®te to Ivan Mak:
+
+ SU> ‚¯¥ç â«ï¥â! ’®«ìª® ¯®ç¥¬y ­  ®¡®¨å áªp¨­è®â å á⮨⠤ â  19 ¬ ï 1998
+ SU> £®¤  ¯p¨ í⮬ ¢ ¤py£®¬ y£«y ­ ¯¨á ­®: Flex Navogator v1.03 (Release
+ SU> data 23.10.1999) Šáâ â¨ ¯p®è«® y¦¥ ¯®ç⨠¯®«â®p  £®¤  á ¬®¬¥­â  ¢ë室 
+ SU> í⮩ ¢¥pᨨ,   §  íâ® ¢p¥¬ï ¬®¦­® ¡ë«® ­ ¯¨á âì y¦¥ ¨ ¢¥pá¨î 2.0.
+ SU> Žâ祣® â ª¨¥ â®p¬®§ ?
+
+19 ¬ ï 1998-£® £®¤ , íâ® á¨á⥬­ ï ¤ â , ãáâ ­ ¢«¨¢ ¥¬ ï ¢ ª®¬¯ìîâ¥p¥,
+¥á«¨ ­¥â CMOS- . ‘ CMOS â ¬ áâ®ï«  ¡ë ⥪ãé ï ¤ â  ¨ ¢p¥¬ï.
+
+€ ¢¥pá¨ï ­¥ ¬¥­ï« áì, ¯®â®¬ã çâ® €­â®­, ª ᮦ «¥­¨î, ᥩç á ¤p㣨¬¨
+¤¥« ¬¨ § ­¨¬ ¥âáï,   ­¥ FN... ­¥ á¯p¨­â¥p®¢áª¨¬¨ :(
+
+        �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)