test(envd): add 136 unit tests across 12 modules

Cover all pure-function modules with inline #[cfg(test)] blocks:
crypto (NIST/RFC 4231 known-answer vectors), auth (SecureToken ops,
signature generation/validation), conntracker (snapshot lifecycle),
execcontext, util (AtomicMax concurrent correctness), http/encoding
(RFC 7231 negotiation), port/conn (/proc/net/tcp parsing),
rpc/entry (format_permissions), and permissions/path (tilde expansion,
ensure_dirs). Add tempfile dev-dep for filesystem tests. Update
Makefile test target to include cargo test.

envd-rs/Cargo.toml

@@ -72,6 +72,9 @@ buffa = "0.3"
 async-stream = "0.3.6"
 mime_guess = "2"
 
+[dev-dependencies]
+tempfile = "3"
+
 [build-dependencies]
 connectrpc-build = "0.3"