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.test.ts

@@ -19,6 +19,8 @@ const UA_R45_MUSL = 'R (4.5.3 x86_64-pc-linux-musl x86_64 linux-musl)';
 const UA_R46_MUSL = 'R (4.6.0 x86_64-pc-linux-musl x86_64 linux-musl)';
 const UA_R45_ALPINE = 'R/4.5.3 R (4.5.3 x86_64-pc-linux-musl x86_64 linux-musl) Alpine Linux 3.24';
 const UA_R45_RESOLUTE = 'R/4.5.3 (Ubuntu 26.04) (aarch64-unknown-linux-gnu aarch64 linux-gnu)';
+const UA_R45_FUTURE_UBUNTU =
+  'R/4.5.3 (Ubuntu 28.04; codename=dynamic-dugong) (aarch64-unknown-linux-gnu aarch64 linux-gnu)';
 const UA_R45_DARWIN = 'R (4.5.1 aarch64-apple-darwin20 aarch64 darwin20)';
 const UA_CURL = 'curl/8.0.1';
 
@@ -150,6 +152,14 @@ Deno.test('rpkgs-router', async (t) => {
       );
     });
 
+    await t.step('resolves a future Ubuntu release from its codename', async () => {
+      const res = await probe('/src/contrib/PACKAGES.gz', UA_R45_FUTURE_UBUNTU);
+      assertEquals(
+        res.location,
+        'https://cran.rpkgs.com/arm64/dynamic-dugong/latest/src/contrib/PACKAGES.gz',
+      );
+    });
+
     await t.step('sends an unidentifiable distro to CRAN', async () => {
       const res = await probe('/src/contrib/PACKAGES.gz', UA_R45_MUSL);
       assertEquals(res.location, 'https://cran.r-project.org/src/contrib/PACKAGES.gz');

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}`;