v4.6.0

Maintenance release — CI gate fixes, code hygiene, and dependency
health. No user-visible behavior changes; this release supersedes the
v4.5.0 artifacts that were published from a commit failing the i18n
and rustfmt gates.

• Fixed: i18n coverage gate — 55 tr() call-site keys that were
  referenced in code but missing from en.ts have been added. Without
  them, those strings rendered their English fallback for every locale
  (including non-English) and could never be translated. The
  i18n-known-missing.json allowlist was regenerated in the explicit
  {missing, stale} object form so stale-key tracking works going
  forward.
• Fixed: cargo fmt gate — 8 engine source files reformatted
  (cargo fmt --all). Purely cosmetic (line wrapping, module
  ordering); zero logic changes.
• Fixed: Dependabot alerts — all open Rust crate vulnerabilities
  (openssl, rustls-webpki, lz4_flex, bytes, rand, time, glib) are now
  resolved in the lockfiles.