test: coverage-guided fuzz targets and a nightly fuzz job

rust/fuzz (cargo-fuzz, nightly, its own workspace with the same IronRDP
patches) has targets for the Telnet IAC parser, the IRC parser + state
machine + mIRC formatter, VNC framebuffer updates and decoders,
~/.ssh/config parsing and the update manifest. They reach the private
parsers through hidden entry points behind nexus-core's 'fuzzing'
feature. A nightly workflow runs each for two minutes and uploads any
crashing input. A first 45 s run of each locally found nothing.

rust/core/Cargo.toml

@@ -3,6 +3,10 @@ name = "nexus-core"
 version.workspace = true
 edition.workspace = true
 
+[features]
+# Exposes parser entry points for rust/fuzz (cargo-fuzz).
+fuzzing = []
+
 [dependencies]
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"