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/rt_test/Makefile

@@ -0,0 +1,5 @@
+# Build rt_test.exe — uses lib/sprinter.lib in TINY memory mode.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := rt_test
+include $(PROJ_ROOT)/examples/example.mk

examples/rt_test/rt_test.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <setjmp.h>
+#include <sprinter_exit.h>
+
+static void bye1(void) { puts("[atexit] bye1 (registered first, runs last)"); }
+static void bye2(void) { puts("[atexit] bye2 (registered second)"); }
+static void bye3(void) { puts("[atexit] bye3 (registered third, runs first)"); }
+
+static jmp_buf env;
+static int     attempt;
+
+static void might_fail(int n)
+{
+    printf("  might_fail(%d) running...\n", n);
+    if (n == 2) {
+        puts("  -> longjmp(env, 42)");
+        longjmp(env, 42);
+        /* not reached */
+    }
+    printf("  might_fail(%d) returned normally\n", n);
+}
+
+int main(void)
+{
+    puts("Sprinter runtime features test:");
+    puts("");
+
+    /* --- atexit registration --- */
+    atexit(bye1);
+    atexit(bye2);
+    atexit(bye3);
+
+    /* --- setjmp / longjmp --- */
+    puts("setjmp/longjmp:");
+    int r = setjmp(env);
+    if (r == 0) {
+        attempt = 1;
+        puts("  first arrival from setjmp (r=0)");
+        might_fail(attempt);
+        attempt = 2;
+        might_fail(attempt);
+        puts("  -- this line is NOT reached");
+    } else {
+        printf("  resumed via longjmp, setjmp returned %d (attempt=%d)\n",
+               r, attempt);
+    }
+
+    /* --- sleep --- */
+    puts("");
+    puts("sleep(5): pausing 5 seconds...");
+    sleep(5);
+    puts("  done.");
+
+    puts("");
+    puts("Press any key, atexit chain will run after.");
+    (void)getchar();
+
+    /* Use exit() to drive the atexit chain (vs returning from main, which
+     * doesn't necessarily go through exit() under SDCC). */
+    exit(0);
+    return 0;
+}