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>

mise.toml

@@ -1,4 +1,7 @@
 [tools]
-"cargo:cargo-nextest" = "latest"
+rust = "1.93"
+"cargo:cargo-deny" = "latest"
+"cargo:cargo-fuzz" = "latest"
 "cargo:cargo-mutants" = "latest"
+"cargo:cargo-nextest" = "latest"
 "cargo:iai-callgrind-runner" = "latest"