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/dir.c

@@ -0,0 +1,68 @@
+/*
+ * ffirst / fnext — directory iteration via ESTEX $19 / $1A.
+ *
+ *   ESTEX F_FIRST  ($19):
+ *     HL = pattern, DE = buffer, A = attribute mask, B = format (0/1)
+ *     CF = err / A = error code
+ *   ESTEX F_NEXT   ($1A):
+ *     DE = same buffer
+ *     CF = err / A = error code
+ *
+ *   We always use format B=1 — 256-byte buffer with NUL-terminated DOS
+ *   "name.ext" name at offset 33.
+ *
+ *   ABI note: ffirst takes uint8_t as its 3rd arg.  SDCC pushes a *single*
+ *   byte for that (via `push af; inc sp`), not two — so the callee must
+ *   pop ret-addr (2 bytes) AND consume the attr byte (`inc sp`) on the way
+ *   out.  Naively `pop bc` would over-eat into the caller's frame.
+ */
+
+#include <dir.h>
+
+int ffirst(const char *pattern, ffblk_t *buf, uint8_t attrib) __naked
+{
+    (void)pattern; (void)buf; (void)attrib;
+    __asm
+        ;; On entry: HL = pattern, DE = buf, [SP+0..1] = ret, [SP+2] = attr.
+        ld    iy, #2
+        add   iy, sp
+        ld    a, 0 (iy)         ; A = attr (read without disturbing SP)
+
+        push  ix
+        ld    bc, #0x0119       ; ESTEX F_FIRST; format: 1 = DOS "name.ext" layout
+        rst   #0x10
+        pop   ix
+
+        pop   hl                ; HL = return address
+        inc   sp                ; consume the 1-byte attr
+
+        jr    c, _ff_err
+        ld    de, #0
+        jp    (hl)
+
+    _ff_err:
+        call  __errno_set
+        ld    de, #-1
+        jp    (hl)
+    __endasm;
+}
+
+int fnext(ffblk_t *buf) __naked
+{
+    (void)buf;
+    __asm
+        ;; HL = buf on entry; ESTEX F_NEXT wants buf in DE.
+        push  ix
+        ex    de, hl
+        ld    c, #0x1A          ; ESTEX F_NEXT
+        rst   #0x10
+        pop   ix
+        jr    c, _fnext_err
+        ld    de, #0
+        ret
+    _fnext_err:
+        call  __errno_set
+        ld    de, #-1
+        ret
+    __endasm;
+}