fix: prevent wallpaper clustering with 16-entry recency buffer (#1606)

## Problem

Custom sleep wallpapers feel repetitive — the same image appearing
multiple times in a short session. Only the single most-recently-shown
index was stored (`uint8_t lastSleepImage`), so on collections of 3–5
images, two-pick cycles were common. On larger collections, any image
could reappear within the next few picks.

## Solution

Add a 16-entry circular recency buffer to `CrossPointState` that
excludes recently shown wallpapers from selection.

**Recency buffer** (`recentSleepImages[16]`, `recentSleepPos`,
`recentSleepFill` — 34 bytes DRAM):
- Tracks the last 16 shown wallpaper indices
- On each pick, rerolls up to 20 times if the candidate was recently
shown
- Window auto-shrinks to `numFiles - 1` for small collections
(guarantees a non-repeat is always possible)
- `isRecentSleep()` clamps to `recentSleepFill` to avoid false positives
on unwritten buffer slots
- State persisted to `state.json` so the buffer survives sleep/wake
cycles

**Migration**:
- Binary (`state.bin`): old `lastSleepImage` field seeded into the new
buffer if valid
- JSON (`state.json`): legacy `lastSleepImage` key detected and seeded
into the buffer when upgrading from older firmware

## Index type fix

`randomFileIndex` upgraded from `uint8_t` to `uint16_t` — silently
truncated for collections larger than 255 wallpapers.

## Repeat probability: before vs after

Chance of seeing a recently-shown image on the next pick. After:
`(numFiles - 16) / numFiles` once collection exceeds the window; 0%
while ≤17.

| Collection | Before | After |
|---|---|---|
| 3 | 50% | 0% |
| 5 | 75% | 0% |
| 10 | 89% | 0% |
| 17 | 94% | 0% |
| 18 | 94% | 6% |
| 20 | 95% | 20% |
| 25 | 96% | 36% |
| 30 | 97% | 47% |
| 50 | 98% | 68% |
| 100 | 99% | 84% |

## Memory impact

| Addition | Size |
|---|---|
| `recentSleepImages[16]` | 32 bytes DRAM |
| `recentSleepPos` + `recentSleepFill` | 2 bytes DRAM |
| **Total** | **34 bytes DRAM** |

## Files changed

- `src/CrossPointState.h` — recency buffer fields + `isRecentSleep()` /
`pushRecentSleep()` declarations
- `src/CrossPointState.cpp` — `isRecentSleep()` / `pushRecentSleep()`
implementations + binary migration path
- `src/JsonSettingsIO.cpp` — JSON serialisation of buffer state + JSON
migration
- `src/activities/boot_sleep/SleepActivity.cpp` — retry loop with
recency check

Co-authored-by: Patryk Radtke <patryk@Patryks-MacBook-Pro.local>

src/CrossPointState.cpp

@@ -5,6 +5,8 @@
 #include <Logging.h>
 #include <Serialization.h>
 
+#include <algorithm>
+
 namespace {
 constexpr uint8_t STATE_FILE_VERSION = 4;
 constexpr char STATE_FILE_BIN[] = "/.crosspoint/state.bin";
@@ -14,6 +16,21 @@ constexpr char STATE_FILE_BAK[] = "/.crosspoint/state.bin.bak";
 
 CrossPointState CrossPointState::instance;
 
+bool CrossPointState::isRecentSleep(uint16_t idx, uint8_t checkCount) const {
+  const uint8_t effectiveCount = std::min(checkCount, recentSleepFill);
+  for (uint8_t i = 0; i < effectiveCount; i++) {
+    const uint8_t slot = (recentSleepPos + SLEEP_RECENT_COUNT - 1 - i) % SLEEP_RECENT_COUNT;
+    if (recentSleepImages[slot] == idx) return true;
+  }
+  return false;
+}
+
+void CrossPointState::pushRecentSleep(uint16_t idx) {
+  recentSleepImages[recentSleepPos] = idx;
+  recentSleepPos = (recentSleepPos + 1) % SLEEP_RECENT_COUNT;
+  if (recentSleepFill < SLEEP_RECENT_COUNT) recentSleepFill++;
+}
+
 bool CrossPointState::saveToFile() const {
   Storage.mkdir("/.crosspoint");
   return JsonSettingsIO::saveState(*this, STATE_FILE_JSON);
@@ -60,9 +77,11 @@ bool CrossPointState::loadFromBinaryFile() {
 
   serialization::readString(inputFile, openEpubPath);
   if (version >= 2) {
-    serialization::readPod(inputFile, lastSleepImage);
-  } else {
-    lastSleepImage = UINT8_MAX;
+    uint8_t legacyLastSleep = UINT8_MAX;
+    serialization::readPod(inputFile, legacyLastSleep);
+    if (legacyLastSleep != UINT8_MAX) {
+      pushRecentSleep(static_cast<uint16_t>(legacyLastSleep));
+    }
   }
 
   if (version >= 3) {