Tracks interaction indices for sequential cassette matching.

Sequential matching is enabled when:

• sequential: true is passed to with_cassette/3
• template: [...] is passed (templates imply sequential matching)

When sequential matching is enabled, requests match interactions in order (request 1 → interaction 0, request 2 → interaction 1, etc.) instead of first-match. This is essential for:

• Identical requests expecting different responses (polling, state changes)
• Templated cassettes where multiple requests have the same structure
• Nested with_cassette calls using the same cassette name

Two Modes

Default Mode (Process Dictionary)

For single-process tests (the common case), session state is stored in the process dictionary. This is simple and requires no setup:

# Explicit sequential matching
with_cassette("test", [sequential: true], fn plug ->
  Req.get!("/status")  # Matches interaction 0
  Req.get!("/status")  # Matches interaction 1
end)

# Templates automatically enable sequential
with_cassette("test", [template: [preset: :common]], fn plug ->
  Req.post!(...)  # Matches interaction 0
  Req.post!(...)  # Matches interaction 1
end)

Shared Session Mode (Agent)

For cross-process tests where HTTP requests are made from spawned processes (Task.async, GenServer, etc.), you must explicitly create a shared session:

session = ReqCassette.start_shared_session()
try do
  with_cassette("test", [session: session, sequential: true], fn plug ->
    Task.async(fn ->
      Req.post!(..., plug: plug)  # Works across processes!
    end) |> Task.await()
  end)
after
  ReqCassette.end_shared_session(session)
end

The shared session uses an Agent process for cross-process state sharing. All operations are serialized through the Agent, guaranteeing consistency.