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/posix_time.c

@@ -0,0 +1,143 @@
+/*
+ * posix_time.c — minimal POSIX <time.h> implementation on top of
+ * getdatetime() (ESTEX SYSTIME $21).
+ *
+ * SDCC's z80.lib bundles time/localtime/mktime AND _RtcRead in a
+ * single time.rel module, so the user can't override _RtcRead from a
+ * separate object — overriding triggers a "multiple definition"
+ * linker error.  We sidestep that by implementing the whole POSIX time
+ * API ourselves; the linker then never pulls SDCC's time.rel.
+ *
+ * The epoch is Unix (1970-01-01 00:00:00).  No timezone support —
+ * gmtime and localtime are identical.  No DST.
+ */
+
+#include <stdio.h>
+#include <time.h>
+
+static const unsigned char mdays[12] = {
+    31,28,31,30,31,30,31,31,30,31,30,31
+};
+
+static const char *const dnames[7] = {
+    "Sun","Mon","Tue","Wed","Thu","Fri","Sat"
+};
+static const char *const mnames[12] = {
+    "Jan","Feb","Mar","Apr","May","Jun",
+    "Jul","Aug","Sep","Oct","Nov","Dec"
+};
+
+static int is_leap(unsigned int y)
+{
+    return (y % 4 == 0 && y % 100 != 0) || (y % 400 == 0);
+}
+
+/* Days elapsed from 1970-01-01 to (y, 1, 1). */
+static unsigned long year_days(unsigned int y)
+{
+    unsigned long d = 0;
+    for (unsigned int i = 1970; i < y; i++)
+        d += is_leap(i) ? 366 : 365;
+    return d;
+}
+
+/* Days from Jan 1 to month start (1-based month input). */
+static unsigned int month_days(unsigned int y, unsigned int m)
+{
+    unsigned int d = 0;
+    for (unsigned int i = 0; i < m - 1; i++) d += mdays[i];
+    if (m > 2 && is_leap(y)) d++;
+    return d;
+}
+
+time_t time(time_t *t)
+{
+    datetime_t dt;
+    getdatetime(&dt);
+    unsigned long days = year_days(dt.year)
+                       + month_days(dt.year, dt.month)
+                       + (dt.day - 1);
+    time_t epoch = days * 86400UL
+                 + (unsigned long)dt.hour   * 3600UL
+                 + (unsigned long)dt.minute * 60UL
+                 + dt.second;
+    if (t) *t = epoch;
+    return epoch;
+}
+
+/* localtime and gmtime share one static buffer — caller copies if
+ * needed across further calls (matches POSIX behaviour). */
+static struct tm tm_buf;
+
+struct tm *gmtime(time_t *timep)
+{
+    unsigned long sec = *timep;
+    tm_buf.tm_sec  = (unsigned char)(sec % 60);  sec /= 60;
+    tm_buf.tm_min  = (unsigned char)(sec % 60);  sec /= 60;
+    tm_buf.tm_hour = (unsigned char)(sec % 24);  sec /= 24;
+    /* sec is now days since 1970-01-01 (Thursday). */
+    tm_buf.tm_wday = (unsigned char)((4 + sec) % 7);
+    /* find year */
+    unsigned int y = 1970;
+    unsigned long days = sec;
+    while (days >= (unsigned long)(is_leap(y) ? 366 : 365)) {
+        days -= is_leap(y) ? 366 : 365;
+        y++;
+    }
+    tm_buf.tm_year = (int)y - 1900;
+    tm_buf.tm_yday = (int)days;
+    /* find month/day */
+    unsigned int m = 0;
+    while (m < 12) {
+        unsigned int dim = mdays[m] + ((m == 1) && is_leap(y) ? 1u : 0u);
+        if (days < dim) break;
+        days -= dim;
+        m++;
+    }
+    tm_buf.tm_mon  = (unsigned char)m;
+    tm_buf.tm_mday = (unsigned char)(days + 1);
+    tm_buf.tm_isdst = 0;
+    tm_buf.tm_hundredth = 0;
+    return &tm_buf;
+}
+
+struct tm *localtime(time_t *timep)
+{
+    return gmtime(timep);   /* no timezone */
+}
+
+time_t mktime(struct tm *tm)
+{
+    unsigned int y = (unsigned int)(tm->tm_year + 1900);
+    unsigned long days = year_days(y)
+                       + month_days(y, (unsigned int)tm->tm_mon + 1)
+                       + (unsigned int)(tm->tm_mday - 1);
+    time_t epoch = days * 86400UL
+                 + (unsigned long)tm->tm_hour * 3600UL
+                 + (unsigned long)tm->tm_min  * 60UL
+                 + tm->tm_sec;
+    /* Backfill wday/yday so callers can inspect them. */
+    tm->tm_wday = (unsigned char)((4 + days) % 7);
+    tm->tm_yday = (int)month_days(y, (unsigned int)tm->tm_mon + 1)
+                + (tm->tm_mday - 1);
+    return epoch;
+}
+
+/* "Day Mon DD HH:MM:SS YYYY\n" — 25 chars + NUL. */
+static char asctime_buf[26];
+
+char *asctime(struct tm *tm)
+{
+    sprintf(asctime_buf, "%s %s %2d %02d:%02d:%02d %d\n",
+            dnames[tm->tm_wday % 7],
+            mnames[tm->tm_mon  % 12],
+            tm->tm_mday,
+            tm->tm_hour, tm->tm_min, tm->tm_sec,
+            tm->tm_year + 1900);
+    return asctime_buf;
+}
+
+char *ctime(time_t *timep)
+{
+    return asctime(localtime(timep));
+}