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/read.c

@@ -0,0 +1,67 @@
+/*
+ * read / write — bulk transfer through ESTEX file handles.
+ *
+ *   ESTEX READ  ($13) / WRITE ($14):
+ *      A = handle, HL = buffer, DE = byte count
+ *      → DE = bytes actually transferred, CF = err with code in A.
+ *
+ * SDCC __sdcccall(1) for 3-arg int functions uses callee-pops for the
+ * stack-passed argument; this implementation mirrors the pattern used
+ * by SDCC's own z80.lib _memset.
+ *
+ * On error: sets errno, returns -1.
+ */
+
+#include <unistd.h>
+
+int read(int fd, void *buf, size_t n) __naked
+{
+    (void)fd; (void)buf; (void)n;
+    __asm
+        pop   iy                       ; IY = return address
+        pop   bc                       ; BC = n  (stack arg)
+        ld    a, l                     ; A = handle
+        ex    de, hl                   ; HL = buf
+        ld    d, b
+        ld    e, c                     ; DE = n
+
+        push  ix
+        push  iy                       ; preserve return addr across RST
+        ld    c, #0x13                 ; ESTEX READ
+        rst   #0x10
+        pop   iy
+        pop   ix
+        jr    c, _read_err
+        ;; DE already holds count read.
+        jp    (iy)
+    _read_err:
+        call  __errno_set
+        ld    de, #-1
+        jp    (iy)
+    __endasm;
+}
+
+int write(int fd, const void *buf, size_t n) __naked
+{
+    (void)fd; (void)buf; (void)n;
+    __asm
+        pop   iy
+        pop   bc
+        ld    a, l
+        ex    de, hl
+        ld    d, b
+        ld    e, c
+        push  ix
+        push  iy
+        ld    c, #0x14
+        rst   #0x10
+        pop   iy
+        pop   ix
+        jr    c, _write_err
+        jp    (iy)
+    _write_err:
+        call  __errno_set
+        ld    de, #-1
+        jp    (iy)
+    __endasm;
+}