Finalize v0.4.0 release content: CHANGELOG, release notes, README

Prepare the source tree for the v0.4.0 release cut, leaving the version
at 0.4.0-dev (the version bump rides a separate release-candidate
commit).

- CHANGELOG: backfill the missing entry for the route-class transit
  counters and fipstop routing-tab reorg, then reorganize the Unreleased
  block from a flat per-commit list into topic-grouped subsections
  mirroring the 0.3.0 entry (coalescing interim fixes into net-effect
  descriptions without dropping technical detail), and stamp it as
  [0.4.0] - 2026-06-21 with a fresh empty Unreleased block. The date is
  provisional and reconfirmed at the final tag.
- Release notes: refresh both RELEASE-NOTES.md and the versioned archive
  (kept byte-identical) to cover the OpenWrt .apk packaging, the Nix
  flake, the macOS self-traffic checksum fix (#117), and the route-class
  transit counters.
- README: bump the status badge to v0.4.0 so it matches the prose.

Claude-Session: https://claude.ai/code/session_01A2pYfSypNmmG4HyHwZuLex

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-v0.4.0--dev-green.svg)](#status--roadmap)
+[![Status](https://img.shields.io/badge/status-v0.4.0-green.svg)](#status--roadmap)
 
 A self-organizing encrypted mesh network built on Nostr identities,
 capable of operating over arbitrary transports without central