Add media sources and the collect runner, alongside the old parsers

Third step: the collector becomes wireable. Both catalogs coexist for exactly
this one step, so ParseViewModel and every existing test stay green while the
new domain is proven.

IMediaSource reuses the closed-generic trick ITextParser used, and for the same
reason - the container cannot resolve an open generic as IEnumerable<T>, so
adding a source stays a one-line registration. Its input is a MediaQuery rather
than text, because a source that walks a paginated listing needs an endpoint and
a cursor, not a string.

Sources discover; they do not download. That split is why UrlListSource lives in
the domain with no network at all, and why everything hard about fetching lives
in one place instead of once per source.

The catalog takes an explicit default id. Left to alphabetical order the landing
source would be the network one, so the app would open behind the proxy gate
before the user had asked for anything.

The runner decouples discovery from downloading with a bounded channel - a
listing of two hundred thousand items must not materialise because the workers
are slower than the source - and owns its workers, waiting for them even when
cancelled. Without that a stopped run keeps writing to the store after the page
has said it stopped.

The own-service listing is read leniently: the service on the other end is the
user's own and should not have to be rewritten to match a schema we invented, so
both a bare array of addresses and an object with items and a cursor work.

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

src/AvParser.Core/DependencyInjection/CoreServiceCollectionExtensions.cs

@@ -1,3 +1,5 @@
+using AvParser.Core.Collecting;
+using AvParser.Core.Collecting.Sources;
 using AvParser.Core.Parsing;
 using AvParser.Core.Parsing.Samples;
 using Microsoft.Extensions.DependencyInjection;
@@ -22,6 +24,15 @@ public static class CoreServiceCollectionExtensions
         services.AddSingleton<ITextParser, KeyValueTextParser>();
         services.AddSingleton<IParserCatalog, ParserCatalog>();
 
+        services.AddSingleton<IMediaSource, UrlListSource>();
+
         return services;
     }
+
+    /// <summary>Id of the source the collector opens on.</summary>
+    /// <remarks>
+    /// Named rather than left to alphabetical order, which would land on the network source and
+    /// open the page behind the proxy gate before the user has asked for anything.
+    /// </remarks>
+    public const string DefaultMediaSourceId = "url-list";
 }