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/sprn2k77.txt

@@ -0,0 +1,22 @@
+*** Area: ZX.SPECTRUM                             Date: 20 Jun 03 23:34:20
+*** From: Vladimir Karpenko (2:5020/826.855)
+*** To  : Kirill Frolov
+*** Subj: ‘¨«ì­® ¨§¢¥­ïîáì.
+
+Hello Kirill
+KF> H¥¬¥¤«¥­­® ­ ¦¬¨ ­  RESET, Vladimir S. Stepanov!
+
+KF> On Thu, 19 Jun 03 03:35:15 +0400, Vladimir S. Stepanov wrote:
+
+MT>>> “¦¥ ᮢᥬ ᪮p® ¢ë©¤¥â ¢ ᢥâ RGB2SVGA-ª®­¢¥pâ¥p. ˆ ¬®¦­® ¡ã¤¥â
+MT>>> ‘¯¥ª ª ᮢp¥¬¥­­ë¬ ¬®­¨ª ¬ ¯®¤ª«îç âì, ª®¬ã ­ ¤®.
+VSS>>  € ¢®â íâ® ¡ë«® ¡ë pã«¥§­®...
+
+KF>    € âë § ©¤¨ ­  http://www.sprinter.com. ’®«ìª® ⥡¥ 業  ­¥ ¯®­p ¢¨âáï.
+
+www.petersplus.com! € 業  ­¨¦¥ 祬 ã ª ï:)
+
+[ZX][Sprinter registered developer][rw1p2][NedoPC][A600HD]
+Bye
+--- FIPS/2001 <build 01.10.07>
+ * Origin: „¥¤ Œ á⤠© ¨ £«îª¨ (2:5020/826.855)