chore: track Cargo.lock, gitignore voice recordings

souveraine is a binary — committing Cargo.lock gives reproducible
builds when installing on another machine and a stable dependency set
for the audit. Voice-input test recordings (voice-recording-*.mp3) are
transient and now ignored.

.gitignore

@@ -1,7 +1,6 @@
 # Rust
 /target/
 **/*.rs.bk
-Cargo.lock
 
 # IDE
 .vscode/
@@ -42,6 +41,9 @@ lcov.info
 local.toml
 config.local.toml
 
+# Voice-input test recordings — transient, stay local
+voice-recording-*.mp3
+
 # Memory repos (should be external)
 .memer/
 **/.memer/