truncateBanner sliced by byte offset, so a multibyte utf-8 rune landing
across the limit was cut mid-sequence and emitted invalid utf-8. shodan
banners routinely carry non-ascii bytes, so this corrupted logged and
printed output. count runes instead so the cut lands on a boundary.
rolls the (c) 2022-2025 banner to 2022-2026 across all go files, the
startup banner in sif.go, and the header-check workflow's expected
format. comment-only, nothing else changes.
