4-Bit Up Counter

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

Behaviour:

• Reset (i_reset='1'): o_cpt ← "0000".
• Rising edge of i_clk:

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

- if i_en='0' → o_cpt is held.

*Asynchronous active-high reset.*