Add Belgian peak demand registers (1-0:1.4.0 / 1-0:1.6.0) to slimmelezer-be.yaml

[snauwaertc]

Hi Marcel,

(Issues are disabled on this repo, so proposing this directly as a PR.)

First off - thanks for the SlimmeLezer. I've only had it running on my Fluvius meter for a week, but it's been flawless so far.

What this adds: since the introduction of the capaciteitstarief in Flanders, the quarter-hour peak is the number every Belgian user cares about. The meter computes and publishes these registers on P1, and ESPHome's dsmr component already supports them - this PR exposes them in slimmelezer-be.yaml:

• active_energy_import_current_average_demand - the meter's live quarter-hour average (OBIS 1-0:1.4.0)
• active_energy_import_maximum_demand_running_month - the official running-month maximum that Fluvius bills on (1-0:1.6.0)
• active_energy_import_maximum_demand_last_13_months - 13-month history (0-0:98.1.0)

No template math or utility_meter workarounds needed on the HA side, and immune to Wi-Fi gaps.

Tested on a SlimmeLezer (ESP8266 d1_mini, firmware 2.0) against a Fluvius S211 (FLU5\...), compiled with ESPHome 2026.4.1: all three report correctly (cross-checked the month peak against a manually computed value - matched within rounding), and existing entity IDs are untouched.

These OBIS codes are Belgium-only (NL meters don't emit them), which is why this targets slimmelezer-be.yaml.