fix: DS3231 temperature 0.25°C steps; add: clear_alarm1/2 helpers

[c0de111]

Summary

• Corrects DS3231 temperature conversion to 0.25°C resolution using proper 10‑bit sign‑extended value.
• Adds ds3231_clear_alarm1() and ds3231_clear_alarm2() helpers (prototypes + implementations).

Changes

• libraries/ds3231/ds3231.c: replace incorrect temp math with sign‑extend + 0.25f scale; add clear_alarm1/2 implementations.
• libraries/ds3231/ds3231.h: declare clear_alarm1/2 prototypes.
• Total: 2 files, 27 additions, 2 deletions.

Details

• Temperature: combine MSB/LSB, right‑shift by 6, sign‑extend 10 bits, multiply by 0.25f for correct quarter‑degree granularity.
• Alarms: read DS3231_CONTROL_STATUS_REG, clear A1F/A2F bits (0/1), write back to acknowledge alarms.

Testing

• Build and flash; observe temperature changes in ~0.25°C increments near ambient.
• Manually trigger A1/A2; call ds3231_clear_alarm1/2(); re‑read status to confirm flags cleared; no regressions in RTC/timekeeping.

Compatibility

• Non‑breaking: temp fix is internal; new alarm clear APIs are additive.