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/sprn2k57.txt

@@ -0,0 +1,56 @@
+*** Area: ZX.SPECTRUM                             Date:  8 Jul 03 17:00:54
+*** From: Kirill Frolov (2:5030/827.2)
+*** To  : Vladimir Karpenko
+*** Subj: � §«¨ç­ë¥ ¯p®¥ªâë ¤«ï ZX
+
+H¥¬¥¤«¥­­® ­ ¦¬¨ ­  RESET, Vladimir Karpenko!
+
+On Mon, 07 Jul 03 22:24:51 +0400, Vladimir Karpenko wrote:
+
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+ VA>> ¯®pâ¨p®¢ ­­®£® á 客 ᮢâ ... :-))
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+
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+
+ VK> € ¤ «ìè¥ çâ®? H¥â ª®­æ¥¯âã «ì­®£® ª®«ìæ  (á) ⮢. H¥¬®!
+
+   “ H¥¬® ⮦¥ ­¨ç¥£® ­¥â.
+
+--- [ZX]
+ * Origin: ­  ¯æ ¬ë áâ ¢¨¬ ªà¥áâ, speccy ¯®ª®«¥­ì¥ next! (2:5030/827.2)
+
+*** Area: ZX.SPECTRUM                             Date:  9 Jul 03 23:24:44
+*** From: Vladimir Karpenko (2:5020/826.855)
+*** To  : Kirill Frolov
+*** Subj: � §«¨ç­ë¥ ¯p®¥ªâë ¤«ï ZX
+
+Hello Kirill
+
+VA>>> ‘®¬­¥¢ îáì... ˆ­ ç¥ ¡ë ­  á¯p¨­â¥p¥ 㦥 ¤ ¢­® ¡ë«¨ ¡ë â®­­ë 
+VA>>> ¯®pâ¨p®¢ ­­®£® á 客 ᮢâ ... :-))
+VK>> € ­ ä¨£:) Hã ¢®â ¡ã¥â ⥡¥ ¤¥áïâì  p娢 â®p®¢ ¨ âp¨¤æ âì p¥¤ ªâ®p®¢?
+
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+KF> ­¥â.
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+KF> ¯p ªâ¨ç¥áª¨ ¡¥á¯®«¥§­ëå á¯p¨­â¥p®¢áª¨å ¯p®£p ¬¬. ‘® á¯p¨­â¥p  ¤ ¦¥ ­¥ 
+KF> 㤠«®áì-¡ë ­ ¯¨á âì ¢ íâã íåã ¡¥§ ᯥªâp㬮¢áª®© Lara Croft.
+
+Œ®¦­® á ��‘ª¨:) “ á¯p¨­â¥p  ¥áâì â¥p¬¨­ «ª ! �« ª Š â!
+
+[ZX][Sprinter registered developer][rw1p2][NedoPC]
+Bye
+--- FIPS/2001 <build 01.10.07>
+ * Origin: Žè¨¡ª  áâ ­®¢¨âáï ®è¨¡ª®©, ª®£¤  p®¦¤ ¥âáï ª ª ¨áâ¨
+(2:5020/826.855)