feat: add prerelease RPM builds from upstream main branch

Poll upstream main branch HEAD alongside release tags. When a new commit
is detected, build and publish prerelease RPMs to a separate unstable
repo at rpm.lair.cafe/fedora/$releasever/$basearch/unstable/.

RPM versioning uses the Fedora snapshot convention (e.g.
0.8.1-0.1.20260511git1a2b3c4.fc43) so stable releases automatically
supersede any installed prerelease.

- RPM spec: conditional Release field via mistralrs_prerelease define
- poll-upstream.yml: new check-prerelease job fetches main HEAD + Cargo.toml version
- build-prerelease.yml: new workflow for commit-based builds without --locked
- UI: fetch both stable/unstable manifests, show channel badges, add
  unstable repo setup instructions to home page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Home.tsx

@@ -10,6 +10,13 @@ enabled=1
 gpgcheck=1
 gpgkey=${GPG_KEY_URL}`;
 
+const UNSTABLE_REPO_FILE = `[lair-cafe-unstable]
+name=lair.cafe RPM Repository (unstable)
+baseurl=https://rpm.lair.cafe/fedora/$releasever/$basearch/unstable/
+enabled=0
+gpgcheck=1
+gpgkey=${GPG_KEY_URL}`;
+
 export function Home() {
   return (
     <>
@@ -43,6 +50,50 @@ export function Home() {
             </Card.Body>
           </Card>
         </Col>
+
+        <Col lg={12}>
+          <Card>
+            <Card.Body>
+              <Card.Title>Unstable (prerelease) packages</Card.Title>
+              <p>
+                Unstable packages are built automatically from the latest
+                upstream <code>main</code> branch commit. They use the
+                next release version from <code>Cargo.toml</code> with a
+                prerelease suffix (e.g.{" "}
+                <code>0.8.1-0.1.20260511git1a2b3c4</code>). When the
+                upstream version is officially released, the stable package
+                will automatically supersede any installed prerelease.
+              </p>
+
+              <h6 className="mt-4">Add the unstable repository</h6>
+              <p className="text-body-secondary">
+                The unstable repo is disabled by default. Add it alongside the
+                stable repo:
+              </p>
+              <CodeBlock language="bash">
+                {`sudo dnf config-manager addrepo --from-repofile=/dev/stdin <<'EOF'\n${UNSTABLE_REPO_FILE}\nEOF`}
+              </CodeBlock>
+
+              <h6 className="mt-4">
+                Install or update from unstable
+              </h6>
+              <CodeBlock language="bash">
+                {`sudo dnf --enablerepo=lair-cafe-unstable install mistralrs-cuda13`}
+              </CodeBlock>
+
+              <h6 className="mt-4">
+                Pin to stable
+              </h6>
+              <p className="text-body-secondary">
+                If you have the unstable repo enabled and want to stay on
+                stable releases, exclude prerelease versions:
+              </p>
+              <CodeBlock language="bash">
+                {`sudo dnf --disablerepo=lair-cafe-unstable update mistralrs-cuda13`}
+              </CodeBlock>
+            </Card.Body>
+          </Card>
+        </Col>
       </Row>
     </>
   );