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/SPRN2K46.TXT

@@ -0,0 +1,32 @@
+*** Area: ZX.SPECTRUM                             Date: 27 Apr 01  2:51:38
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Sergey Krinitsin
+*** Subj: Sprinter News
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Sergey!
+
+  <Tuesday April 24 2001> <00:33> Sergey Krinitsin wr®te to Ivan Mak:
+
+ IM>> ‚ ¤ ­­ë© ¬®¬¥­â, ¨á¯®«ì§ã¥âáï  ªá¥«¥p â®p, ª®â®pë©
+ IM>> ¢ë¯®«­ï¥â 7 ¬«­.
+ IM>> ¢ë¡®p®ª ¨§ Ž‡“ ¢ ᥪ㭤ã (íâ® ¯p¥¤¥« Ž‡“).
+ SK>                              ^^^^^^^^^^^^^^^^^
+ SK> Š ª®¥ Ž‡“ ¢ë ¨á¯®«ì§ã¥â¥? �“'誨 ç⮫¨?
+
+72-pin SIMM (¯p® EDO/FPM p á᪠§ë¢ âì ­¥ ­ ¤®, §­ î)...
+
+ IM>> �p®æ¥áá®p Z80 ⮫쪮 «¨èì ã¯p ¢«ï¥â í⨬ ¯p®æ¥áᮬ, ¨ ­ 
+ SK>               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ SK> ’ ª CPU ¢á¥¦¥ Z80? H   «ìâ¥à¥ ⮫쪮 ᮯà®æ¥áá®à?
+
+„ . “¦¥ £®¢®p¨«®áì ®¡ í⮬ ¨ ­¥ p §...
+Sprinter ¨¬¥¥â _¯p®æ¥áá®p Z80 +  ªá¥«¥p â®p ¢ �‹Œ_
+
+        �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)