chore: make repo an Arduino collection with WeatherPredictor project

Flatten so the repo root is the Arduino sketchbook; the project lives in
WeatherPredictor/. Add a collection index README at the root and move the
detailed project README into WeatherPredictor/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/history.cpp

@@ -0,0 +1,76 @@
+#include <LittleFS.h>
+#include "config.h"
+#include "history.h"
+
+static Sample s_buf[HISTORY_SIZE];
+static int    s_head  = 0;   // next write position
+static int    s_count = 0;
+
+void historyBegin() { s_head = 0; s_count = 0; }
+
+void historyAdd(uint32_t epoch, float mslHpa, float tempC) {
+  s_buf[s_head] = Sample{ epoch, mslHpa, tempC };
+  s_head = (s_head + 1) % HISTORY_SIZE;
+  if (s_count < HISTORY_SIZE) s_count++;
+}
+
+int historyCount() { return s_count; }
+
+Sample historyGet(int i) {
+  int start = (s_head - s_count + HISTORY_SIZE) % HISTORY_SIZE;
+  return s_buf[(start + i) % HISTORY_SIZE];
+}
+
+Sample historyLatest() { return historyGet(s_count - 1); }
+
+bool historyTrendDelta(float& outDelta) {
+  if (s_count < 2) return false;
+  Sample latest = historyLatest();
+  if (latest.epoch < 10800UL) return false;
+  uint32_t target = latest.epoch - 10800UL;   // 3 h earlier
+  // Pick the newest sample at or before `target`. Scan all samples (do not
+  // assume epochs are strictly increasing: an NTP correction can shift them).
+  int idx = -1;
+  uint32_t bestEpoch = 0;
+  for (int i = 0; i < s_count; i++) {
+    Sample s = historyGet(i);
+    if (s.epoch <= target && s.epoch >= bestEpoch) { bestEpoch = s.epoch; idx = i; }
+  }
+  if (idx < 0) return false;
+  Sample past = historyGet(idx);
+  if (latest.epoch - past.epoch < 9000UL) return false;  // need >= 2.5 h span
+  outDelta = latest.mslHpa - past.mslHpa;
+  return true;
+}
+
+static const char* HPATH = "/history.dat";
+
+// Binary format: [int32 count][count * Sample], stored oldest-first.
+bool historySave() {
+  File f = LittleFS.open(HPATH, "w");
+  if (!f) return false;
+  int32_t n = s_count;
+  f.write((const uint8_t*)&n, sizeof(n));
+  for (int i = 0; i < s_count; i++) {
+    Sample s = historyGet(i);
+    f.write((const uint8_t*)&s, sizeof(s));
+  }
+  f.close();
+  return true;
+}
+
+bool historyLoad() {
+  File f = LittleFS.open(HPATH, "r");
+  if (!f) return false;
+  int32_t n = 0;
+  if (f.read((uint8_t*)&n, sizeof(n)) != sizeof(n)) { f.close(); return false; }
+  if (n < 0 || n > HISTORY_SIZE) { f.close(); return false; }
+  s_head = 0; s_count = 0;
+  for (int i = 0; i < n; i++) {
+    Sample s;
+    if (f.read((uint8_t*)&s, sizeof(s)) != sizeof(s)) break;
+    historyAdd(s.epoch, s.mslHpa, s.tempC);
+  }
+  f.close();
+  return true;
+}