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/sprn2k74s.txt

@@ -0,0 +1,24 @@
+*** Area: ZX.SPECTRUM                             Date:  8 Jun 03 23:12:42
+*** From: Vladimir Karpenko (2:5020/826.855)
+*** To  : Vitaly Mayatskih
+*** Subj: � §«¨ç­ë¥ ¯p®¥ªâë ¤«ï ZX
+
+Hello Vitaly
+VM> ‡¤®p®¢¥­­®¥ ⥡¥ ¯p¨¢¥â¨é¥, Vladimir!
+
+VM>  07 ˆî­ 03 15:32, Vladimir Karpenko ¯¨è¥â Sergey Zuev á«¥¤ãî饥:
+
+SZ>>> ’ ª¦¥ ¨­â¥p¥á­ë ­®¢ë¥ p §p ¡®âª¨ â ª¨å ¬ áèâ ¡®¢ ¯®¤ ZX
+SZ>>> ( ¯¯ p â­ë¥ ¨ ¯p®£p ¬¬­ë¥).
+VK>> Sprinter 97
+VK>> Sprinter 2000
+VK>> Sprinter 2000s
+VM>         € 祬 ¯®á«¥¤­¨© ®â ¯p¥¤¯®á«¥¤­¥£® ®â«¨ç ¥âáï?
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+¢¨¤¥® ª®­­¥ªâ®p ¢ë¢¥¤¥­® +12‚ ¢¬¥áâ® +5‚.
+
+[ZX][Sprinter registered developer][rw1p2][NedoPC][A600HD]
+Bye
+--- FIPS/2001 <build 01.10.07>
+ * Origin: ‘®¤®¬ ¨ £¥¬®pp®© - ¤¢¥ ¢¥é¨ ­¥á®¢¬¥áâ­ë¥ (2:5020/826.855)