Add v0.4.1 changelog entry and release notes

Back-fill the changelog for the six user-facing changes on this line
since v0.4.0: the antipoison FPR cap default, the bloom probe and
secp256k1 context performance work, the coordinate-cache and path-MTU
fixes, and the removed parent_switched counter. The Unreleased section
was empty, so all six entries are new.

Write the v0.4.1 release notes and mirror them to the root copy. The
notes lead with the FPR cap change and state plainly that this is the
second raise of that default in two releases, that it buys headroom
rather than fixing the underlying fixed-filter constraint, and that the
structural remedy is the v2 filter work. They also flag the rolling
upgrade window, where an upgraded and a not-yet-upgraded node can
disagree about mesh size, and point consumers of parent_switched at
parent_switches.

Bump the README status badge and the roadmap prose together so the two
agree.

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--dev-green.svg)](#status--roadmap)
+[![Status](https://img.shields.io/badge/status-v0.4.1-green.svg)](#status--roadmap)
 
 A self-organizing encrypted mesh network built on Nostr identities,
 capable of operating over arbitrary transports without central
@@ -210,8 +210,8 @@ testing/      Docker-based integration test harnesses + chaos simulation
 
 ## Status & roadmap
 
-FIPS is at **v0.4.1-dev** on the `maint` branch.
-[v0.4.0](https://github.com/jmcorgan/fips/releases/tag/v0.4.0) has
+FIPS is at **v0.4.1** 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
 UDP, TCP, Ethernet, Tor, Nym, and Bluetooth on a global, public test