Add core engine: tarot + astrology reading generator with CLI and i18n

Plain-C, dependency-free engine that produces a personalized daily
Celtic Cross tarot spread and natal-chart/transit astrology reading,
plus a standalone CLI (dist/deck-engine) and run.sh wrapper. Includes
English/German output via a simple key=value translation format, and
docs/diagrams describing the architecture and I/O.

engine/src/rng.c

@@ -0,0 +1,38 @@
+#include "rng.h"
+
+uint64_t rng_seed_from_string(const char *seed) {
+  uint64_t hash = 0xcbf29ce484222325ULL; /* FNV offset basis */
+  for (const unsigned char *p = (const unsigned char *)seed; *p; p++) {
+    hash ^= (uint64_t)*p;
+    hash *= 0x100000001b3ULL; /* FNV prime */
+  }
+  return hash;
+}
+
+void rng_init(RngState *rng, uint64_t seed) {
+  /* splitmix64 requires a nonzero-friendly state; any seed works, but
+   * avoid the degenerate all-zero stream for the empty string. */
+  rng->state = seed ? seed : 0x9e3779b97f4a7c15ULL;
+}
+
+uint64_t rng_next_u64(RngState *rng) {
+  uint64_t z = (rng->state += 0x9e3779b97f4a7c15ULL);
+  z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9ULL;
+  z = (z ^ (z >> 27)) * 0x94d049bb133111ebULL;
+  return z ^ (z >> 31);
+}
+
+uint32_t rng_next_bounded(RngState *rng, uint32_t bound) {
+  /* Rejection sampling against the largest multiple of `bound` that fits
+   * in 32 bits, so every outcome in [0, bound) is equally likely. `limit`
+   * must stay 64-bit: when `bound` evenly divides 2^32 (e.g. bound == 2,
+   * used for every reversed-card coin flip), the true limit is 2^32
+   * itself, which truncates to 0 in a uint32_t and turns the loop below
+   * into an infinite one. */
+  uint64_t limit = 0x100000000ULL - (0x100000000ULL % bound);
+  uint64_t value;
+  do {
+    value = rng_next_u64(rng) & 0xffffffffULL;
+  } while (value >= limit);
+  return (uint32_t)(value % bound);
+}