fix: Fix bootloop logging crash (#1357)

## Summary

* **What is the goal of this PR?** On a cold boot (or after a crash that
corrupts RTC RAM), logHead contains garbage. Then addToLogRingBuffer
does: ``strncpy(logMessages[logHead], message, MAX_ENTRY_LEN - 1); ``
With garbage logHead, this computes a completely invalid address. The %
MAX_LOG_LINES guard on line 16 only runs after the bad store, which is
too late. The fix is to clamp logHead before use.

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_ (did use claude
for the magic hash value)

lib/Logging/Logging.h

@@ -57,6 +57,11 @@ void logPrintf(const char* level, const char* origin, const char* format, ...);
 
 std::string getLastLogs();
 void clearLastLogs();
+// Validates the RTC log state (magic word + logHead range). Returns true if
+// corruption was detected (magic mismatch or logHead out of range), meaning
+// logMessages is untrusted garbage. Callers should call clearLastLogs() when
+// this returns true so getLastLogs() does not dump corrupt data into crash reports.
+bool sanitizeLogHead();
 
 class MySerialImpl : public Print {
  public: