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>

runtime/heap_top.s

@@ -0,0 +1,22 @@
+;; ----------------------------------------------------------------------
+;; heap_top.s — defines the absolute upper bound of the heap.
+;;
+;; This file is built PER-PROGRAM (not bundled into sprinter.lib) so the
+;; value of ___sdcc_heap_end can be overridden without re-assembling the
+;; library or the user's crt0.  sprinter-cc may regenerate this file in
+;; its working dir with a custom value when `--stack-size N` is passed;
+;; otherwise the default below applies.
+;;
+;; malloc takes &___sdcc_heap_end as the upper bound of the free-list,
+;; so the symbol's address (== the equate value) IS the heap ceiling.
+;;
+;;   0xBFFE  — stack top (init SP in crt0)
+;;   0xBB00  — heap top (default)  →  ~1278 bytes reserved for the stack
+;;   ...     — heap grows up to here from end-of-BSS
+;;   end-of-BSS  — heap start
+;; ----------------------------------------------------------------------
+
+    .module sprinter_heap_top
+
+    ___sdcc_heap_end = 0xBB00
+    .globl  ___sdcc_heap_end