Process-wide registry of shared RepoHandles keyed by URL.

Solves Chris's "scenario 3" from the design review: 100 LiveView sessions searching the same repository should share ONE cache, not each clone the repo independently.

First caller to get_or_start/2 for a given URL pays the clone + prefetch cost. Subsequent callers for the same URL receive the same handle — the cache is warm.

API

# Atomic get-or-start. Blocks until the repo is cloned and
# the handle is ready. Returns the existing handle on
# subsequent calls for the same URL.
{:ok, handle} = Exgit.RepoRegistry.get_or_start(url)

# Check without starting.
case Exgit.RepoRegistry.lookup(url) do
  {:ok, handle} -> handle
  :error -> nil
end

# Stop and remove a handle.
:ok = Exgit.RepoRegistry.stop(url)

# Introspection.
Exgit.RepoRegistry.count()
Exgit.RepoRegistry.list()

Concurrency

Concurrent calls to get_or_start/2 for the SAME URL serialize at the registry GenServer; only one clone happens. Concurrent calls for DIFFERENT URLs do NOT block each other in any user-visible way (each hits a fast Registry.lookup; the slow path serializes per-URL via a short-lived Mutex-equivalent check inside the server).

Options

Per-URL options supplied to get_or_start/2 (e.g. :lazy, :filter) are applied by the FIRST caller for that URL. Subsequent callers' options are ignored (with a telemetry event when they differ). Consumers who need different per-user configs should use Exgit.clone/2 directly without the registry.

Lifecycle

Handles started by the registry are linked to the registry process. If the registry dies, all handles die. If a handle dies (crash, explicit stop), the registry removes it from its map and the next get_or_start/2 will start a fresh clone.

Start the registry as a named, singleton GenServer (typically under the consumer's supervision tree). If not started, all API calls error with :not_started.