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/attrprob/attrprob.c

@@ -0,0 +1,79 @@
+#include <stdio.h>
+#include <conio.h>
+#include <stdint.h>
+
+/*
+ * attr_probe — visualise every value of the ESTEX attribute byte.
+ *
+ * Layout:
+ *   row 2   16 cells: low nibble 0..F, high nibble 0   (varying fg)
+ *   row 4   16 cells: low nibble 7, high nibble 0..F   (varying bg)
+ *   row 6   bit-7 toggle test: 8 pairs comparing N with N|0x80
+ *   row 8   full 16x16 grid: bg outer, fg inner — every attribute byte
+ *   row 26  RDCHAR readback of a known cell
+ *
+ * Each cell is drawn with wrchar() so the attribute is set exactly,
+ * independent of ESTEX's hidden "current" attribute used by PCHARS.
+ */
+
+int main(void)
+{
+    clrscr();
+
+    gotoxy(20, 0); cputs("== attr probe ==");
+
+    /* Row 2: vary fg (low nibble), bg = 0 (black). */
+    gotoxy(0, 2);  cputs("fg 0..F /bg0:");
+    for (int i = 0; i < 16; i++) {
+        wrchar(15 + i, 2, 'A' + i, (uint8_t)i);
+    }
+
+    /* Row 4: vary bg (high nibble), fg = 7 (light gray). */
+    gotoxy(0, 4);  cputs("bg 0..F /fg7:");
+    for (int i = 0; i < 16; i++) {
+        wrchar(15 + i, 4, 'A' + i, (uint8_t)((i << 4) | 0x07));
+    }
+
+    /* Row 6: compare attr N with N|0x80 — bit 7 may be blink or extra bg. */
+    gotoxy(0, 6);  cputs("bit7: N|N|0x80 pairs (1F,2E,3D,4C,5B,6A,79,F0):");
+    {
+        static const uint8_t probes[] = { 0x1F, 0x2E, 0x3D, 0x4C,
+                                          0x5B, 0x6A, 0x79, 0xF0 };
+        for (int i = 0; i < 8; i++) {
+            wrchar(2 + i*4,     7, '*', probes[i]);
+            wrchar(2 + i*4 + 1, 7, '*', probes[i] | 0x80);
+            /* hex labels under each pair */
+            gotoxy(2 + i*4, 8);
+            printf("%02X", probes[i]);
+        }
+    }
+
+    /* Row 10..25: full 16x16 grid of every attr byte 0x00..0xFF.
+     * Cell (col, row) = (col_idx + offset, row_idx + 10) holds char '#'
+     * with attr = (row_idx << 4) | col_idx. */
+    gotoxy(0, 10); cputs("Full 16x16 grid (row=bg, col=fg):");
+    for (int row = 0; row < 16; row++) {
+        for (int col = 0; col < 16; col++) {
+            uint8_t attr = (uint8_t)((row << 4) | col);
+            wrchar(20 + col, 11 + row, '#', attr);
+        }
+        /* label rows */
+        gotoxy(15, 11 + row);
+        printf("%X:", row);
+    }
+    gotoxy(20, 27);
+    cputs("0123456789ABCDEF  (fg)");
+
+    /* RDCHAR readback test. */
+    {
+        uint16_t got = rdchar(15, 2);
+        gotoxy(0, 29);
+        printf("rdchar(15,2) -> ch=0x%02X attr=0x%02X (expected 'A'=0x41 attr=0x00)",
+               got & 0xFF, (got >> 8) & 0xFF);
+    }
+
+    gotoxy(0, 31);
+    cputs("press any key");
+    getch();
+    return 0;
+}