Add SyncPlay stats-for-nerds page, manual sync button, and release pipeline

Phase 0 scaffold plus everything since: quality negotiation, host-relay,
and active drift correction were each built and reverted after real bugs
in live use (see PLAN.md for the full account of each). What remains is
read-only SyncPlay observability (live drift/playback stats page) and a
one-shot manual "sync me to group" action, plus Phase 5 packaging/release
tooling (build.yaml, manifest.json generator, Gitea Actions release
workflow) to distribute it as a proper plugin repository.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

dev/update_manifest.py

@@ -0,0 +1,85 @@
+#!/usr/bin/env python3
+"""Appends (or replaces) this build's version entry in manifest.json, using build.yaml
+for the plugin-level metadata (guid, name, description, etc.) and changelog. Run by
+dev/package-release.sh; not meant to be invoked directly outside that flow."""
+import argparse
+import datetime
+import json
+import re
+
+
+def parse_build_yaml(path):
+    # build.yaml is simple enough to not need a real YAML parser dependency.
+    text = open(path, encoding="utf-8").read()
+    fields = {}
+    for key in ("name", "guid", "targetAbi", "framework", "owner", "overview", "category"):
+        m = re.search(rf'^{key}:\s*"([^"]*)"', text, re.MULTILINE)
+        if m:
+            fields[key] = m.group(1)
+
+    def block(key):
+        m = re.search(rf'^{key}:\s*>\s*\n((?:  .*\n?)+)', text, re.MULTILINE)
+        if not m:
+            return ""
+        lines = [line[2:].rstrip() for line in m.group(1).splitlines()]
+        return " ".join(line for line in lines if line).strip()
+
+    fields["description"] = block("description")
+    fields["changelog"] = block("changelog")
+    return fields
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--build-yaml", required=True)
+    parser.add_argument("--manifest", required=True)
+    parser.add_argument("--version", required=True)
+    parser.add_argument("--checksum", required=True)
+    parser.add_argument("--source-url", required=True)
+    args = parser.parse_args()
+
+    build = parse_build_yaml(args.build_yaml)
+
+    try:
+        with open(args.manifest, encoding="utf-8") as f:
+            manifest = json.load(f)
+    except FileNotFoundError:
+        manifest = []
+
+    entry = next((p for p in manifest if p.get("guid") == build["guid"]), None)
+    if entry is None:
+        entry = {
+            "guid": build["guid"],
+            "name": build["name"],
+            "description": build["description"],
+            "overview": build["overview"],
+            "owner": build["owner"],
+            "category": build["category"],
+            "versions": [],
+        }
+        manifest.append(entry)
+    else:
+        entry["description"] = build["description"]
+        entry["overview"] = build["overview"]
+        entry["owner"] = build["owner"]
+        entry["category"] = build["category"]
+
+    entry["versions"] = [v for v in entry["versions"] if v["version"] != args.version]
+    entry["versions"].insert(0, {
+        "version": args.version,
+        "changelog": build["changelog"],
+        "targetAbi": build["targetAbi"],
+        "sourceUrl": args.source_url,
+        "checksum": args.checksum,
+        "timestamp": datetime.datetime.now(datetime.timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ"),
+    })
+
+    with open(args.manifest, "w", encoding="utf-8") as f:
+        json.dump(manifest, f, indent=4)
+        f.write("\n")
+
+    print(f"Updated {args.manifest} with version {args.version}")
+
+
+if __name__ == "__main__":
+    main()