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/videomode_raw.c

@@ -0,0 +1,46 @@
+/*
+ * videomode_raw.c — low-level ESTEX SETVMOD / GETVMOD ($50 / $51).
+ *
+ * Plain getters/setters with NO mode-class validation.  Used by both
+ * conio (text-validated public API) and gfx (graphics modes).  Lives
+ * in its own .c so a pure graphics program does not pull in the entire
+ * conio module to switch modes.
+ *
+ * Public conio functions in conio.c wrap these with a text-mode check;
+ * gfx_init / gfx_done in gfx_core.c call them directly.
+ */
+
+#include <stdint.h>
+#include <errno.h>
+
+uint8_t _videomode_raw_get(void) __naked
+{
+    __asm
+        push  ix
+        ld    c, #0x51              ; ESTEX GETVMOD
+        rst   #0x10
+        pop   ix
+        ;; uint8_t returns in A — ESTEX already put mode there.
+        ret
+    __endasm;
+}
+
+int _videomode_raw_set(uint8_t mode) __naked
+{
+    (void)mode;
+    __asm
+        ;; SDCC __sdcccall(1) passes uint8_t in A — leave it there.
+        push  ix
+        ld    bc, #0x0050           ; ESTEX SETVMOD (B=0 (page), C=0x50)
+        rst   #0x10
+        jr    c, _vmr_err
+        ld    de, #0
+        pop   ix
+        ret
+    _vmr_err:
+        call  __errno_set
+        ld    de, #-1
+        pop   ix
+        ret
+    __endasm;
+}