feat(cdn): derive Ubuntu routes from codenames (#174)

## Motivation

Ubuntu release codenames cannot be derived from version numbers, so the edge router currently needs a code change for every Ubuntu release.

The generic repository setup can already read `VERSION_CODENAME` from `/etc/os-release` and send it in the user agent.

## Changes

- Prefer a strictly validated `codename=<name>` token for Ubuntu slot routing.
- Retain the existing version-to-codename table for clients using the previous user-agent format.
- Add a future Ubuntu 28.04 regression case whose codename does not exist in the middleware table.

## User-agent format

`R/4.6.1 (Ubuntu 26.04; codename=resolute) (aarch64-unknown-linux-gnu)`

## Verification

- `just edge-test`, 1 test with 16 steps passed.
- `git diff --check`

Reviewed-on: #174

edge/rpkgs-router.ts

@@ -58,6 +58,7 @@ const MACOS_BIN_REGEX =
 const RHEL_REGEX = /(almalinux|rocky)[^\d]*(\d+)/i;
 
 const UBUNTU_REGEX = /Ubuntu ([\d.]+)/i;
+const UBUNTU_CODENAME_REGEX = /Ubuntu [\d.]+;\s*codename=([a-z][a-z0-9-]*)/i;
 const UBUNTU_CODENAMES: Record<string, string> = {
   '26.04': 'resolute',
   '24.04': 'noble',
@@ -137,6 +138,11 @@ function parseSlot(userAgent: string): string | null {
 
   const ubuntu = userAgent.match(UBUNTU_REGEX);
   if (ubuntu) {
+    const codenameMatch = userAgent.match(UBUNTU_CODENAME_REGEX);
+    if (codenameMatch) {
+      return `${arch}/${codenameMatch[1].toLowerCase()}`;
+    }
+
     const codename = UBUNTU_CODENAMES[ubuntu[1]];
     if (codename) {
       return `${arch}/${codename}`;