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.

Makefile

@@ -115,6 +115,7 @@ vet:
 
 test:
 	go test -race -v ./internal/...
+	cd envd-rs && cargo test
 
 test-integration:
 	go test -race -v -tags=integration ./tests/integration/...