feat: add Puppet Forge remote type (#44)

## Summary

- Adds \`package: puppet\` for proxying Puppet Forge (forgeapi.puppet.com)
- \`remote/puppet.py\` rewrites JSON responses: absolute forge URLs → proxy URLs, and relative \`/v3/files/\` \`file_uri\` paths → absolute proxy URLs. g10k uses Go's \`url.ResolveReference\`, so an absolute \`file_uri\` overrides the base URL entirely — tarballs are fetched directly from the proxy without a second hop
- Built-in mutable patterns: \`^v3/modules/\` and \`^v3/releases\` (module metadata); tarballs at \`v3/files/\` are configured as immutable via \`immutable_patterns\`
- 9 new tests covering mutable detection, URL rewriting (relative \`file_uri\` and absolute forge URLs), content-type, tarball pass-through, and pattern blocking

## Client configuration

**g10k config file** (\`forge_base_url\` at root level):
\`\`\`yaml
cachedir: /tmp/g10k
forge_base_url: https://artifacts.example.com/api/v1/remote/puppet-forge
sources:
  control:
    remote: git@git.example.com:puppet/control.git
    basedir: /etc/puppetlabs/code/environments
\`\`\`

**Puppetfile** (\`forge.baseUrl\` directive, works with \`-puppetfile\` mode):
\`\`\`ruby
forge.baseUrl https://artifacts.example.com/api/v1/remote/puppet-forge

mod 'puppetlabs-stdlib', '9.7.0'
\`\`\`

## Test plan

- [x] 331 unit tests pass (\`make test\`)
- [x] End-to-end: g10k 0.9.10 on AlmaLinux 9 via \`forge_base_url\` — stdlib 9.7.0, inifile 6.2.0, concat 9.1.0 installed; proxy logs confirm cache MISS → fetch → ADD for metadata and tarballs
- [x] End-to-end: \`forge.baseUrl\` Puppetfile directive with \`-puppetfile\` mode — same result

Reviewed-on: #44

tests/conftest.py

@@ -112,6 +112,12 @@ TEST_REMOTES = {
             "immutable_patterns": [r"\.tgz$"],
             "cache": {"immutable_ttl": 0, "mutable_ttl": 1800},
         },
+        "puppet-test": {
+            "base_url": "https://forgeapi.puppet.com",
+            "package": "puppet",
+            "immutable_patterns": [r"^v3/files/.*\.tar\.gz$"],
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 600},
+        },
     },
     "locals": {
         "local-test": {