v0.1.0-alpha.6: sparse Character snapshot defaults

Benchmark death counts now apply the protocol's zero defaults to sparse Character CREATE snapshots. New login and replacement CREATE packets clear the previous Character observation. Partial VALUES packets still leave absent fields unchanged.

Includes PR #8 and the dead/unknown reporting from alpha.5. The correction passed parent review, 65 tests, strict clippy, release builds, and Linux, macOS and coverage CI.