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/malloc/Makefile

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

examples/malloc/malloc.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/*
+ * Exercises SDCC's malloc / free.  The library pulls in heap.rel + malloc.rel
+ * from z80.lib automatically and inits the heap from crt0's _GSINIT chain.
+ * Default heap size is 1023 bytes — see docs/TODO.md for plans to grow it.
+ */
+
+int main(void)
+{
+    puts("malloc test:");
+
+    /* small allocations */
+    char *a = (char *)malloc(20);
+    char *b = (char *)malloc(40);
+    char *c = (char *)malloc(60);
+
+    printf("  a = %p\n", a);
+    printf("  b = %p\n", b);
+    printf("  c = %p\n", c);
+
+    if (a && b && c) {
+        strcpy(a, "first");
+        strcpy(b, "second-medium-length");
+        strcpy(c, "third-and-the-longest-of-them-all");
+        printf("  a: \"%s\"\n", a);
+        printf("  b: \"%s\"\n", b);
+        printf("  c: \"%s\"\n", c);
+    } else {
+        puts("  one of the allocations FAILED");
+    }
+
+    /* free b, alloc smaller — should fit in the b-shaped hole */
+    free(b);
+    char *d = (char *)malloc(15);
+    printf("  after free(b), d = %p (expected near b)\n", d);
+    if (d) {
+        strcpy(d, "reused");
+        printf("  d: \"%s\"\n", d);
+    }
+
+    /* drain a larger allocation to see when malloc actually says no */
+    int total = 0;
+    int count = 0;
+    for (;;) {
+        void *p = malloc(64);
+        if (!p) break;
+        total += 64;
+        count++;
+    }
+    printf("  alloc'd %d x 64-byte blocks before failure (total %d bytes)\n",
+           count, total);
+
+    free(a);
+    free(c);
+    free(d);
+    puts("");
+    puts("Press any key to exit.");
+    (void)getchar();
+    return 0;
+}