fix: wipe credentials in the connect path and keep release builds out of dumps and debuggers

- Credential callbacks (SecretFn) now return Zeroizing<String>, as do the
  session-only/pending caches they read and the RDP client's password,
  so Nexus's own copies are wiped; a plain String copy is made only
  where a library API needs one (SSH password auth, IRC, VNC, CredSSP).
- Release builds disable core dumps (RLIMIT_CORE 0) and, on Linux, mark
  the process non-dumpable (PR_SET_DUMPABLE 0), which also blocks ptrace
  attach and /proc/<pid>/mem reads by other processes of the same user.
  Debug builds are unchanged so they stay debuggable. Verified on the
  release binary: core limit 0, /proc/<pid>/{environ,mem} root-owned and
  unreadable, local shells (including cwd capture) still work.

Closes the remaining parts of SEC-20261012-007 that the app controls;
egui's internal text buffers remain outside its reach.

rust/Cargo.lock

@@ -3452,6 +3452,7 @@ dependencies = [
  "egui",
  "egui-phosphor",
  "image",
+ "libc",
  "nexus-core",
  "parking_lot",
  "regex",