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/SPRN2K29.TXT

@@ -0,0 +1,31 @@
+*** Area: ZX.SPECTRUM                             Date: 14 Apr 01 22:15:39
+*** From: Ivan Mak (2:5030/529.24)
+*** To  : Andreas Kaiser
+*** Subj: ‘¯à¨­â¥à ¨ â.¯.
+
+
+        �ਢ¥âáâ¢ãî ‚ á, Andreas!
+
+  <Tuesday April 10 2001> <22:28> Andreas Kaiser wr®te to All:
+
+ AK> Šáâ â¨, ˆ¢ ­, ï ¯à®¢¥à¨« ¢ ©â ­  ¯à®æ, ª ª âë £®¢®à¨«, ¨¤ñâ,   ¬ëè 
+ AK> ­¥ ¯ è¥â.
+
+‚ ©â â® â ¬ ­¨ ¯p¨ 祬... H㦭® ¯p®¢¥p¨âì ⮫쪮 ¯p®å®¦¤¥­¨¥ +12, -5 ¢®«ìâ ­ 
+¬ëèì ¨ ᨣ­ « á ¬ëè¨ ç¥p¥§ ¨­¢¥pâ®p ­  ¯p®æ¥áá®p (¬ëèì ¯p¨ ¯p®¢¥pª¥ 襢¥«¨âì
+­ á®, çâ® ¡ë ¡ë«® ¢¨¤­®).
+Hã ¨ ¥é¥ ⠪⮢ãî ç áâ®âã, çâ® ¨¤¥â ­  ¯®á«¥¤®¢ â¥«ì­ë© ¯®pâ...
+
+ AK> �த «¨ ¡à ª®¢ ­ãî ¯« âã, ¡«¨­,   ï ¨§¤ «¥ª  ¯à¨¥å « ;))
+
+�«¨­, ª ª®© ¡p ª?! Ÿ ¥¥ «¨ç­® ¯p®¢¥pï« ¯¥p¥¤ í⨬...
+€ â® çâ® â ¬ ¯p®¢®¤ , â ª íâ® ­  ¢á¥å ¯« â å ®­¨ â ª¨¥... ’ë ¦¥ ¨å ¢¨¤¥«...
+
+
+        �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)