chore: audit cleanup, CI, cargo-deny, encode fuzz target (0.3.4)

Simplify encoder exit branch (−41 B code size), add GitHub Actions CI
with MSRV matrix, cargo-deny with permissive-only license policy,
encode-only fuzz target, and feature coverage across test gates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ucobs"
-version = "0.3.3"
+version = "0.3.4"
 edition = "2021"
 rust-version = "1.93"
 license = "MIT"