SendspinKit: add the minimal public seams needed for conformance

[balloob]

Problem

SendspinKit still needs library-side seams to let conformance use the real SendspinClient instead of a bespoke protocol client.

The conformance rule should be strict here: if the library cannot exercise a scenario, the test should expose that gap. The adapter should not fill in missing behavior by manually constructing/parsing protocol messages.

Why this matters

For server-initiated and binary-verification scenarios, conformance needs a minimal way to:

• hand an already-accepted connection into the real client implementation
• observe the library's actual handshake / stream / disconnect behavior
• verify artwork/audio delivery without reimplementing the protocol in the adapter

Local exploration also showed that once conformance is routed through the real client, actual library issues become visible instead of being hidden by bespoke adapter code. That is exactly what we want.

Scope

Keep this limited to the minimum surface needed to make testing possible. Do not use this as a reason to add unrelated new features.

Tracking

• Agree the minimum public seams needed for conformance (for example transport injection and observer hooks)
• Add those minimal public seams in SendspinKit
• Update the conformance adapter to use the real SendspinClient
• Let remaining unsupported or incorrect behavior fail as library bugs rather than adapter workarounds
• Re-run the aiosendspin -> SendspinKit conformance slice and record the remaining failures/passes

[balloob-travel]

Implementation plan

The important rule for SendspinKit is the same one we have been applying elsewhere: conformance should exercise the real SendspinClient, not a custom transport/protocol client built inside the adapter. If a scenario fails once routed through the real library, that is useful signal and should remain visible.

Recommended plan:

1. Add only the minimum public seams needed to route conformance through SendspinClient.
   Based on the current adapter needs, that likely means:

   • a transport abstraction the real client can consume
   • a way to connect using an already-accepted inbound connection for server-initiated scenarios
   • a small observer/callback surface for raw lifecycle events the adapter must summarize

2. Keep these seams narrow and library-owned.
   The adapter should not parse or construct protocol messages itself. The library should continue to own:

   • handshake parsing
   • server-state parsing
   • stream-start parsing
   • audio/artwork frame handling
   • disconnect semantics

3. Expose observer hooks for verification, not alternate protocol implementations.
   The likely hooks needed are:

   • server/hello observed
   • server/state observed
   • stream/start observed
   • audio chunk observed
   • artwork frame observed
   • disconnect observed

   These should be read-only observation points around the real client behavior, not a second low-level API that bypasses it.

4. Add only the command surface required for existing conformance scenarios.
   If controller-role conformance requires a public way to send controller commands through SendspinClient, add only that minimal path. Do not use this issue to broaden the API beyond what is needed to test existing scenarios.

5. Validate through focused library tests before changing conformance.
   Add tests for:

   • transport injection / connect-via-transport behavior
   • event ordering for server/hello, stream/start, and disconnect
   • audio/artwork observer callbacks receiving the same bytes the client processes
   • controller command send path if added

6. Update the conformance adapter to become thin glue only.
   After the library seams exist, the adapter should do little more than:

   • parse CLI args
   • wire a transport/listener
   • subscribe to public callbacks
   • write the scenario summary

7. Accept real library failures after the migration.
   Once the adapter is using the real SendspinClient, any remaining failures in player, metadata, artwork, or controller scenarios should be treated as library issues unless the adapter is still doing something outside the public SDK surface.

8. Sequence work in two PRs if needed.
   The clean order is:

   • first, library PR in SendspinKit
   • then, conformance PR switching the adapter to those new public seams

Non-goals:

• expanding the product scope of SendspinKit beyond what existing conformance scenarios require
• adding bespoke protocol parsing to conformance to keep the matrix green
• hiding real library shortcomings behind adapter-local code

[balloob]

Fixed in Sendspin/SendspinKit#6