https-dns-proxy: update to 2026.03.18-4

https-dns-proxy: update to 2026.03.18-4

Maintainer: me
Compile tested: x86_64, Dell EMC Edge620, OpenWrt 25.12.1
Run tested: x86_64, Dell EMC Edge620, OpenWrt 25.12.1

Description:
Robustify nftables notrack generation

  - Bump PKG_RELEASE to 4.
  - Add install rule to create /usr/share/nftables.d/ruleset-post.

files/etc/init.d/https-dns-proxy:
  - Check if 'nft' command exists before generating rules.
  - Add robust error handling for directory creation and file writing.
  - Log errors when directory creation or file writing fails.

tests/run_tests.sh:
  - Add test case for 'nft' binary absence, confirming no-op behavior.
  - Add test case for mkdir failure during notrack rule generation.

Signed-off-by: Stan Grishin <redacted>