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/activities/boot_sleep/SleepActivity.cpp

@@ -85,13 +85,16 @@ void SleepActivity::renderCustomSleepScreen() const {
     }
     const auto numFiles = files.size();
     if (numFiles > 0) {
-      // Generate a random number between 1 and numFiles
-      auto randomFileIndex = random(numFiles);
-      // If we picked the same image as last time, reroll
-      while (numFiles > 1 && APP_STATE.lastSleepImage != UINT8_MAX && randomFileIndex == APP_STATE.lastSleepImage) {
-        randomFileIndex = random(numFiles);
+      // Pick a random wallpaper, excluding recently shown ones.
+      // Window: up to SLEEP_RECENT_COUNT entries, capped at numFiles-1.
+      const uint16_t fileCount = static_cast<uint16_t>(std::min(numFiles, static_cast<size_t>(UINT16_MAX)));
+      const uint8_t window =
+          static_cast<uint8_t>(std::min(static_cast<size_t>(APP_STATE.recentSleepFill), numFiles - 1));
+      auto randomFileIndex = static_cast<uint16_t>(random(fileCount));
+      for (uint8_t attempt = 0; attempt < 20 && APP_STATE.isRecentSleep(randomFileIndex, window); attempt++) {
+        randomFileIndex = static_cast<uint16_t>(random(fileCount));
       }
-      APP_STATE.lastSleepImage = randomFileIndex;
+      APP_STATE.pushRecentSleep(randomFileIndex);
       APP_STATE.saveToFile();
       const auto filename = std::string(sleepDir) + "/" + files[randomFileIndex];
       FsFile file;