feat: diagnostic log (tracing)

The app now writes a diagnostic log to nexus.log in the data directory
(0600, rotated at 5 MiB): connection attempts and results, remote
closes, reconnect scheduling, host-key/certificate checks with the
policy applied, and credentials that couldn't be stored (kind only).
Credentials and terminal data are never logged. RUST_LOG sets the level
and also mirrors the log to stderr. The remaining eprintln! diagnostics
in storage and URL handling go through it too.

rust/Cargo.lock

@@ -3225,6 +3225,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "matchers"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d1525a2a28c7f4fa0fc98bb91ae755d1e2d1505079e05539e35bc876b5d65ae9"
+dependencies = [
+ "regex-automata",
+]
+
 [[package]]
 name = "md-5"
 version = "0.10.6"
@@ -3449,6 +3458,8 @@ dependencies = [
  "rfd",
  "serde",
  "serde_json",
+ "tracing",
+ "tracing-subscriber",
  "winresource",
  "zeroize",
 ]
@@ -3483,6 +3494,7 @@ dependencies = [
  "sha2 0.11.0",
  "tiny_http",
  "tokio",
+ "tracing",
  "ureq",
  "uuid",
  "webpki-roots",
@@ -3554,6 +3566,15 @@ dependencies = [
  "minimal-lexical",
 ]
 
+[[package]]
+name = "nu-ansi-term"
+version = "0.50.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7957b9740744892f114936ab4a57b3f487491bbeafaf8083688b16841a4240e5"
+dependencies = [
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "num"
 version = "0.4.3"
@@ -5507,6 +5528,15 @@ dependencies = [
  "sponge-cursor",
 ]
 
+[[package]]
+name = "sharded-slab"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f40ca3c46823713e0d4209592e8d6e826aa57e928f09752619fc696c499637f6"
+dependencies = [
+ "lazy_static",
+]
+
 [[package]]
 name = "shared_library"
 version = "0.1.9"
@@ -5984,6 +6014,15 @@ dependencies = [
  "syn 3.0.7",
 ]
 
+[[package]]
+name = "thread_local"
+version = "1.1.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1ad99c4c6d32803332c548b1af0540b357b3f5fc0be8f6c6bfe8b2e6ae784070"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "tiff"
 version = "0.11.3"
@@ -6198,6 +6237,22 @@ dependencies = [
  "once_cell",
 ]
 
+[[package]]
+name = "tracing-subscriber"
+version = "0.3.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb7f578e5945fb242538965c2d0b04418d38ec25c79d160cd279bf0731c8d319"
+dependencies = [
+ "matchers",
+ "nu-ansi-term",
+ "once_cell",
+ "regex-automata",
+ "sharded-slab",
+ "thread_local",
+ "tracing",
+ "tracing-core",
+]
+
 [[package]]
 name = "type-map"
 version = "0.5.1"