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/ptime/ptime.c

@@ -0,0 +1,44 @@
+/*
+ * ptime — exercise POSIX <time.h>.  All wired through SDCC's z80.lib
+ * time.rel which calls our RtcRead() (bridged to ESTEX SYSTIME).
+ */
+
+#include <stdio.h>
+#include <time.h>
+
+int main(void)
+{
+    /* time() returns seconds since 1970-01-01 (Unix epoch). */
+    time_t now;
+    time(&now);
+    printf("time()      = %lu epoch seconds\n", now);
+
+    /* localtime() decomposes into struct tm. */
+    struct tm *lt = localtime(&now);
+    printf("localtime() = %04d-%02d-%02d %02d:%02d:%02d  (wday=%u)\n",
+           lt->tm_year + 1900,
+           lt->tm_mon  + 1,
+           lt->tm_mday,
+           lt->tm_hour,
+           lt->tm_min,
+           lt->tm_sec,
+           (unsigned)lt->tm_wday);
+
+    /* asctime() formats it as a 25-char string. */
+    printf("asctime()   = %s", asctime(lt));    /* asctime adds \n */
+
+    /* mktime() round-trip — should recover the same epoch. */
+    time_t round = mktime(lt);
+    printf("mktime()    = %lu  (round-trip %s original)\n",
+           round, (round == now ? "==" : "!="));
+
+    /* Native datetime_t API still works alongside. */
+    datetime_t dt;
+    getdatetime(&dt);
+    printf("native      = %04u-%02u-%02u %02u:%02u:%02u  (dow=%u)\n",
+           dt.year, dt.month, dt.day,
+           dt.hour, dt.minute, dt.second,
+           (unsigned)dt.dow);
+
+    return 0;
+}