Open 0.4.2-dev cycle on maint after v0.4.1 release

Bump the version to 0.4.2-dev and update the status badge and prose. The
0.4.x patch line continues; v0.4.1 remains the shipped release it points at.

Follows the shape of the v0.4.0 rollover commit. No maint reset was needed
this time: v0.4.1 was cut from maint, so the tag is already on this line and
it simply continues.

Cargo.lock

@@ -1074,7 +1074,7 @@ checksum = "9844ddc3a6e533d62bba727eb6c28b5d360921d5175e9ff0f1e621a5c590a4d5"
 
 [[package]]
 name = "fips"
-version = "0.4.1"
+version = "0.4.2-dev"
 dependencies = [
  "arc-swap",
  "bech32",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "fips"
-version = "0.4.1"
+version = "0.4.2-dev"
 edition = "2024"
 description = "A distributed, decentralized network routing protocol for mesh nodes connecting over arbitrary transports"
 license = "MIT"

README.md

@@ -3,7 +3,7 @@
 ![banner](docs/logos/fips_banner.png)
 [![License: MIT](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 [![Rust](https://img.shields.io/badge/rust-1.85%2B-orange.svg)](https://www.rust-lang.org/)
-[![Status](https://img.shields.io/badge/status-v0.4.1-green.svg)](#status--roadmap)
+[![Status](https://img.shields.io/badge/status-v0.4.2--dev-green.svg)](#status--roadmap)
 
 A self-organizing encrypted mesh network built on Nostr identities,
 capable of operating over arbitrary transports without central
@@ -210,7 +210,7 @@ testing/      Docker-based integration test harnesses + chaos simulation
 
 ## Status & roadmap
 
-FIPS is at **v0.4.1** on the `maint` branch.
+FIPS is at **v0.4.2-dev** on the `maint` branch.
 [v0.4.1](https://github.com/jmcorgan/fips/releases/tag/v0.4.1) has
 shipped; this line carries patch-level fixes for the 0.4.x series. The
 core protocol works end-to-end over