Add DHCP static reservation read

[MarcLeinenDE]

Adds a small read-only helper for the upstream live-verified normal-admin router/router_get_dhcp_static_ip contract.

Scope:

• client.lan.static_reservations() → router/router_get_dhcp_static_ip
• exact GET routing locked down by offline tests
• complete firmware JSON response is preserved raw because the upstream public contract does not freeze a stable nested response schema
• existing read-only LAN/DNS physical smoke extended with the new helper

Safety/privacy:

• no router_set_dhcp_static_ip
• no DHCP/LAN configuration write
• physical smoke does not print or assert concrete reservation IP/MAC values
• no invented field names or normalization

Physical ACIY.3 evidence (2026-09-08):

• targeted test_lan_dns_reads passed 1/1 in 0.92 s on Python 3.13.5
• exercised existing LAN/DNS reads plus router/router_get_dhcp_static_ip
• no DHCP/LAN write and no concrete reservation IP/MAC values printed

Documentation:

• CHANGELOG.md records helper and physical validation
• docs/dhcp-static-reservations-read.md records scope, privacy and evidence
• no nr2301-api change is needed because this run introduced no new wire semantics; the getter was already upstream LIVE_VERIFIED, ADMIN_OK, READ_OR_LOW_SIDE_EFFECT

Validation:

• initial PR CI run #227: Python 3.10/3.11/3.12/3.13 and package all SUCCESS
• final PR patch reviewed; only intended SDK/helper/test/evidence changes remain