feat: Playnite library sync plugin + exporter

A two-part Punktfunk plugin that syncs the Playnite library into the host
game library as the `playnite` provider:

- exporter/ — a minimal C# Playnite GenericPlugin ("Punktfunk Sync") that
  writes punktfunk-library.json on every library change. Reading Playnite's
  live-locked LiteDB from outside isn't robust, so this adapter runs inside
  Playnite. Builds net462 via reference assemblies; packaged as a .pext.
- src/ — the TS plugin (on @punktfunk/host, rom-manager shape): watches the
  export, embeds covers as data URLs, reconciles via
  PUT /library/provider/playnite, with a console-hosted web UI + standalone
  fallback + CLI. Launch = playnite://playnite/start/<id>, run by the host
  in the interactive session so Playnite performs the real launch.

Verified locally: backend tsc + 23 tests + biome clean, SPA build, C#
exporter build + .pext pack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

exporter/README.md

@@ -0,0 +1,69 @@
+# Punktfunk Sync — Playnite exporter
+
+The Playnite side of [`@punktfunk/plugin-playnite`](../README.md): a minimal
+[GenericPlugin](https://api.playnite.link/docs/tutorials/extensions/genericPlugins.html) that writes
+your library to `punktfunk-library.json` whenever it changes. The Punktfunk plugin on the host reads
+that file and reconciles it into the streaming library.
+
+It exists because Playnite keeps its library database (LiteDB) open and locked while running, so the
+only robust way to read the library — with correct install state, art paths and metadata — is from
+*inside* Playnite. This plugin does exactly that and nothing else: no network, no settings, no UI.
+
+## What it writes
+
+On startup, on every `OnLibraryUpdated`, and (debounced) on any per-game add/update/remove, it writes:
+
+```
+%APPDATA%\Playnite\ExtensionsData\<plugin-guid>\punktfunk-library.json
+```
+
+Shape (kept in lockstep with `../src/export-schema.ts`):
+
+```jsonc
+{
+  "schema": 1,
+  "generatedAt": "2026-07-18T21:00:00Z",
+  "playnite": { "version": "10.x", "mode": "Desktop" },
+  "games": [
+    {
+      "id": "e3b0c442-98fc-1c14-9afb-4c8996fb9242",  // Playnite Guid = reconcile external_id
+      "name": "Hollow Knight",
+      "installed": true,
+      "hidden": false,
+      "source": "Steam",
+      "platforms": ["PC (Windows)"],
+      "cover": "C:\\Users\\me\\AppData\\Roaming\\Playnite\\library\\files\\ab\\cover.jpg",
+      "background": null,
+      "icon": null
+    }
+  ]
+}
+```
+
+Art paths are **absolute** (resolved via `IPlayniteAPI.Database.GetFullFilePath`). Filtering
+(installed-only, hidden, per-source) happens on the Punktfunk side, so the export is the full library.
+
+You can also trigger a write by hand: Playnite **main menu → Punktfunk Sync → Export library to
+Punktfunk now**.
+
+## Install (for users)
+
+Double-click `punktfunk-sync.pext` — Playnite installs it like any add-on — and restart Playnite.
+Grab the `.pext` from the repo's [CI artifacts](https://git.unom.io/unom/punktfunk-plugin-playnite/actions).
+
+## Build
+
+Targets .NET Framework 4.6.2 (Playnite's runtime). Thanks to
+`Microsoft.NETFramework.ReferenceAssemblies` it builds with the plain .NET SDK on any OS — no Windows
+or MSBuild needed:
+
+```sh
+dotnet build PunktfunkSync.csproj -c Release
+# output: bin/Release/PunktfunkSync.dll + extension.yaml (PlayniteSDK.dll is provided by Playnite)
+```
+
+Package the `.pext` (a zip of the two files):
+
+```sh
+( cd bin/Release && zip -j ../../punktfunk-sync.pext extension.yaml PunktfunkSync.dll )
+```