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/assrtest/Makefile

@@ -0,0 +1,3 @@
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := assrtest
+include $(PROJ_ROOT)/examples/example.mk

examples/assrtest/assrtest.c

@@ -0,0 +1,43 @@
+/*
+ * assrtest — exercise <assert.h> from SDCC's z80.lib.
+ *
+ * Tests both the disabled-debug (NDEBUG) form and the active form.
+ * The failing assert at the end prints an error message via printf
+ * (which routes through our putchar) and then halts.
+ */
+
+#include <stdio.h>
+#include <assert.h>
+
+static int divide(int a, int b)
+{
+    assert(b != 0);            /* must succeed for the test cases */
+    return a / b;
+}
+
+int main(int argc, char **argv)
+{
+    (void)argc; (void)argv;
+
+    puts("assrtest: exercising <assert.h>");
+
+    /* These pass. */
+    int q = divide(10, 2);
+    printf("  divide(10, 2) = %d\n", q);
+    assert(q == 5);
+
+    q = divide(-9, 3);
+    printf("  divide(-9, 3) = %d\n", q);
+    assert(q == -3);
+
+    puts("  all passing asserts OK");
+
+    /* Final, failing assert.  Prints "Assert(...) failed in function ..."
+     * then enters an infinite loop (SDCC __assert convention). */
+    puts("  triggering deliberate failing assert (system will hang)...");
+    int zero = 0;
+    (void)divide(1, zero);
+
+    /* unreachable */
+    return 0;
+}