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>

examples/seek/seek.c

@@ -0,0 +1,66 @@
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+/*
+ * Exercises lseek() with offsets that span the 16-bit boundary.
+ * big.txt is 102400 bytes (100 KB); every 256-byte block opens with a
+ * "[off=NNNNNNN] block #BBBB " label, so any seek aligned to 256 lands
+ * on a self-identifying marker.
+ */
+
+static void show_marker(int fd, long off)
+{
+    long pos = lseek(fd, off, SEEK_SET);
+    printf("\nseek %ld -> pos=%ld\n", off, pos);
+
+    char buf[28];
+    int n = read(fd, buf, 27);
+    if (n <= 0) {
+        printf("  read failed: %d\n", n);
+        return;
+    }
+    buf[n] = 0;
+    printf("  bytes: \"%s\"\n", buf);
+}
+
+int main(void)
+{
+    int fd = open("BIG.TXT", O_RDONLY);
+    if (fd < 0) {
+        printf("open failed: %d\n", fd);
+        (void)getchar();
+        return 1;
+    }
+
+    /* Total file size. */
+    long size = lseek(fd, 0L, SEEK_END);
+    printf("file size = %ld bytes\n", size);
+
+    /* Seek inside the first 64 KB (high16 = 0). */
+    show_marker(fd, 0L);
+    show_marker(fd, 256L);
+    show_marker(fd, 32768L);
+
+    /* Right at the 64 KB boundary (offset 0x00010000, high16 = 1). */
+    show_marker(fd, 65536L);
+
+    /* Past 64 KB (high16 still 1, low16 != 0). */
+    show_marker(fd, 65536L + 256L);
+    show_marker(fd, 81920L);
+
+    /* Near end of file (size = 102400 = 0x19000, high16 = 1). */
+    show_marker(fd, 102400L - 256L);
+
+    /* SEEK_CUR: from current position, +256. After the previous SEEK_SET
+     * to 102144, then read of 27 bytes, pos is 102171. +256 -> 102427,
+     * which is past EOF — ESTEX should clamp or error. */
+    long after_cur = lseek(fd, 256L, SEEK_CUR);
+    printf("\nSEEK_CUR +256 -> pos=%ld (past EOF)\n", after_cur);
+
+    close(fd);
+    puts("");
+    puts("Press any key to exit...");
+    (void)getchar();
+    return 0;
+}