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/hooks/usePackages.ts

@@ -1,23 +1,47 @@
 import { useEffect, useState } from "react";
-import type { PackagesManifest } from "../types/packages.ts";
+import type { Channel, PackagesManifest, PackageVersion } from "../types/packages.ts";
 
-const MANIFEST_URL = "/fedora/43/x86_64/packages.json";
+const STABLE_URL = "/fedora/43/x86_64/packages.json";
+const UNSTABLE_URL = "/fedora/43/x86_64/unstable/packages.json";
+
+function tagPackages(
+  manifest: PackagesManifest,
+  channel: Channel,
+): PackageVersion[] {
+  return manifest.packages.map((p) => ({
+    ...p,
+    channel,
+    baseUrl: manifest.baseUrl,
+  }));
+}
 
 export function usePackages() {
-  const [manifest, setManifest] = useState<PackagesManifest | null>(null);
+  const [packages, setPackages] = useState<PackageVersion[]>([]);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
     let cancelled = false;
 
-    fetch(MANIFEST_URL)
-      .then((res) => {
-        if (!res.ok) throw new Error(`HTTP ${res.status}`);
-        return res.json() as Promise<PackagesManifest>;
-      })
-      .then((data) => {
-        if (!cancelled) setManifest(data);
+    const fetchManifest = async (
+      url: string,
+      channel: Channel,
+    ): Promise<PackageVersion[]> => {
+      const res = await fetch(url);
+      if (!res.ok) {
+        if (res.status === 404) return [];
+        throw new Error(`HTTP ${res.status} fetching ${channel} manifest`);
+      }
+      const data = (await res.json()) as PackagesManifest;
+      return tagPackages(data, channel);
+    };
+
+    Promise.all([
+      fetchManifest(STABLE_URL, "stable"),
+      fetchManifest(UNSTABLE_URL, "unstable"),
+    ])
+      .then(([stable, unstable]) => {
+        if (!cancelled) setPackages([...stable, ...unstable]);
       })
       .catch((err: unknown) => {
         if (!cancelled)
@@ -32,5 +56,5 @@ export function usePackages() {
     };
   }, []);
 
-  return { manifest, loading, error };
+  return { packages, loading, error };
 }