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>

libc/stdio/putchar.c

@@ -0,0 +1,42 @@
+/*
+ * putchar — emit one character via ESTEX PUTCHAR ($5B).
+ *
+ * Turbo-C convention: this stdio.h function is the FAST path with NO
+ * attribute control.  Whatever ESTEX has cached for the cursor cell is
+ * used (typically the shell's default colour).  Translates '\n' to
+ * CR LF for C-string semantics.
+ *
+ * For coloured output, use putch() / cputs() / cprintf() from <conio.h>
+ * — those honour textattr / g_text_attr at the cost of being ~10× slower.
+ *
+ * SDCC __sdcccall(1): char arg in L (low byte of HL=int).  Returns the
+ * char in DE (SDCC int return).
+ */
+
+#include <stdio.h>
+
+int putchar(int c) __naked
+{
+    (void)c;
+    __asm
+        ld    a, l                       ; SDCC __sdcccall(1) int → HL
+        push  ix
+        cp    #0x0A
+        jr    nz, _pc_emit
+        ld    a, #0x0D                   ; CR before LF
+        push  af
+        ld    c, #0x5B
+        rst   #0x10
+        pop   af
+        ld    a, #0x0A
+    _pc_emit:
+        push  af
+        ld    c, #0x5B
+        rst   #0x10
+        pop   af
+        pop   ix
+        ld    e, a
+        ld    d, #0
+        ret
+    __endasm;
+}