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/SPRN2K41.TXT

@@ -0,0 +1,44 @@
+*** Area: ZX.SPECTRUM                             Date: 23 Apr 01 23:03:00
+*** From: Alex Goryachev (2:5030/529.56)
+*** To  : Slavka Kalinin
+*** Subj: ‘¯à¨­â¥à ¨ â.¯.
+
+    �ਢ¥âáâ¢ãî, Slavka!
+
+   Friday April 20 2001 Slavka Kalinin -> Alex Goryachev.
+
+ AG>> �« â  Sp2000 ¡ã¤¥â ¢ë¯ã᪠âìáï
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+
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+ SK> ¡ã¤¥â, ‚ƒ ¡ã¤¥â ¢ €«ìâ¥à¥.
+
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+⮫쪮 ¢ €«ìâ¥pã ¨ â¥pï«  íâã ¨­ä®p¬ æ¨î ¯p¨ ¢ëª«î祭¨¨ ¯¨â ­¨ï.
+   Ž¡ê¥¬ �‡“ 㢥«¨ç¥­ ¤® 256Šb ¨ ⥯¥pì ¨á¯®«ì§ã¥âáï ä«íè ¯ ¬ïâì.
+   16 ¡¨â­ë© –€� ¤«ï 2 ª ­ «®¢ áâ¥p¥® §¢ãª .
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+¢¨­ç¥áâ¥p  ¨«¨ CD-ROM .
+   ‘ ¬  ¯« â  ¢ë¯®«­¥­  ­  ¡®«¥¥ ¢ë᮪®¬ â¥å­®«®£¨ç¥áª®¬ ãp®¢­¥ á íªp ­¨pãî饩
+á¥âª®© ¨ § é¨â­ë¬ ¯®ªpë⨥¬.
+
+
+    “¤ ç¨!
+
+    Alex Goryachev.
+
+    [ZX]...[Sprinter]...  Force in flex cource!
+
+---
+ * Origin: PETERS PLUS LTD (2:5030/529.56)