[Enhancement] Properties Library: Selectable CS Pin, Bypass SD.begin(), and Proper LittleFS Support [Testing]

## Enhancement Request: Properties Library [Parent issue] ### Overview Two related improvements are needed for the `Properties` library to make it more flexible and compatible with projects that already manage their own SD or filesystem initialization. --- ### 1. Selectable CS Pin & Ability to Bypass `SD.begin()` **Current Behavior:** Every SD-backed method (`saveToSD`, `loadFromSD`, `save`, `load`, `store`, `storeToXML`, `loadFromXML`, etc.) calls `SD.begin(chipSelect)` internally — on **every single operation**. This means: - If the calling application has already called `SD.begin()`, the library re-initializes the SD card on every call, which can cause conflicts, slow things down, or fail silently. - There is no way to tell the library "I already initialized the SD card, just use it." Additionally, one call site in `Properties.cpp` hard-codes `SD.begin(4)` instead of using the configurable `chipSelect` field, so `setChipSelect()` is not consistently respected. **Requested Changes:** - Add a `setBypassSDBegin(bool bypass)` flag so the library can skip `SD.begin()` when the caller has already initialized the SD card. - Fix the hard-coded `SD.begin(4)` to use `chipSelect` consistently everywhere. - Ensure `setChipSelect()` takes effect in all `SD.begin()` call sites. **Example Usage:** ```cpp // User initializes SD themselves SD.begin(10); Properties props; props.setChipSelect(10); props.setBypassSDBegin(true); // Don't re-initialize SD props.loadFromSD("/config.properties"); 2. Proper LittleFS Support (Dynamic Filesystem Selection) Current Behavior: A separate LittleFSProperties class exists, but the main Properties class is tightly coupled to SD only. Users wanting LittleFS must use a completely different class with a different API. Requested Changes: Add dynamic filesystem selection to the Properties class via an enum + setter (e.g. setFilesystem(FS_SD) / setFilesystem(FS_LITTLEFS)). All file operation methods (save, load, store, and all format-specific variants) should transparently route to the chosen filesystem. This makes the existing LittleFSProperties class redundant for most use cases. Properties props; // Use LittleFS props.setFilesystem(Properties::FS_LITTLEFS); props.load("/config.properties"); // Or use SD Properties props2; props2.setFilesystem(Properties::FS_SD); props2.setChipSelect(10); props2.load("/config.properties"); Notes The author has existing code ready to implement these fixes. Backward compatibility should be preserved — default behavior should remain SD with SD.begin() called automatically, so existing sketches are not broken. --- **🔗 Duplicate of:** #138 *This issue has been identified as a duplicate and linked to the parent issue above.*