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/gfx/gfx_raw_common.c

@@ -0,0 +1,44 @@
+/*
+ * gfx_raw_common.c — W3 page mapping primitives shared by every gfx mode.
+ *
+ * Composite primitives (gfx_line / gfx_rect / gfx_fill_rect / ...) bracket
+ * their inner loop with one `_gfx_w3_video_begin()` / `_gfx_w3_video_end()`
+ * pair and call the `*_raw` variants inside, so the W3 dance is paid once
+ * per drawing operation instead of once per pixel.  Single-shot wrappers
+ * (gfx_putpixel, gfx_hline, ...) wrap the same way for their single call.
+ *
+ * Begin disables interrupts, saves the current W3 page byte, then maps
+ * `_gfx_bank` (the current video bank, 0x50..0x5F).  End restores the
+ * saved page and re-enables interrupts.
+ *
+ * NOT re-entrant — saving the previous W3 byte in a static is safe only
+ * because GFX runs with interrupts off between begin and end.
+ */
+
+#include <stdint.h>
+
+extern uint8_t _gfx_bank;     /* current W3 video bank (gfx_core.c) */
+
+static uint8_t _gfx_saved_w3;
+
+void _gfx_w3_video_begin(void) __naked
+{
+    __asm
+        di
+        in    a, (#0xE2)
+        ld    (__gfx_saved_w3), a
+        ld    a, (__gfx_bank)
+        out   (#0xE2), a
+        ret
+    __endasm;
+}
+
+void _gfx_w3_video_end(void) __naked
+{
+    __asm
+        ld    a, (__gfx_saved_w3)
+        out   (#0xE2), a
+        ei
+        ret
+    __endasm;
+}