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/io/_errno_set.c

@@ -0,0 +1,40 @@
+/*
+ * _errno_set — set `errno` from an ESTEX error code (0..255 in A).
+ *
+ * Replaces the inline pattern
+ *      ld    (_errno),    a
+ *      xor   a, a
+ *      ld    (_errno+1),  a            ; 7 bytes per error path
+ * with a single
+ *      call  __errno_set                ; 3 bytes per error path
+ *
+ * Saves ~4 bytes at every libc error handler that converts an ESTEX
+ * code into the C-side `errno`.  Helper itself is 7 bytes; with 10+
+ * error paths in our libc the size win is net-positive.
+ *
+ * ABI:
+ *   in:    A  = ESTEX error code (0..255)
+ *   out:   HL = A (zero-extended); errno fully overwritten so a prior
+ *               large value (e.g. errno = -1) can't leak its high byte.
+ *   clobbers: HL, AF flags.  Caller must not depend on HL afterwards.
+ *
+ * Defensive 16-bit store — see chat 2026-06-02: if anyone ever assigns
+ * errno via C (`errno = -1`), the high byte becomes 0xFF, and a partial
+ * 8-bit write here would leave that 0xFF in place.  Always writing the
+ * full word keeps errno honest regardless of who set it last.
+ */
+
+void _errno_set(unsigned char code) __naked
+{
+    (void)code;
+    __asm
+        ;; __sdcccall(1): single uint8_t arg already in A.
+        ;; Write the two bytes separately so HL/BC/DE/IX/IY remain
+        ;; untouched.  Only A is clobbered: it is the input register,
+        ;; and ABI does not require preserving it across a void call.
+        ld    (_errno), a            ; low byte = code
+        xor   a, a
+        ld    (_errno+1), a          ; high byte = 0
+        ret
+    __endasm;
+}