4-Bit Down Counter

4-bit down counter. Walks 15 → 0 then wraps to 15.

Behaviour:

• Reset (i_reset='1'): o_cpt ← "1111" (15).
• Rising edge of i_clk:

- if i_en='1' → o_cpt ← o_cpt − 1 (automatic 0 → 15 wrap),

- if i_en='0' → o_cpt is held.

*Asynchronous active-high reset.*