feat(sendspin-rs): add a server adapter for the PCM player scenarios

[davidgraeff]

What

Adds a real Rust server adapter for sendspin-rs, which previously had
only a client adapter (its server role was a placeholder). This lets the
matrix exercise sendspin-rs as a server against real clients.

• adapters/sendspin-rs/server/ drives the crate's server API
  (ServerListener / dial_client + Group): decodes the FLAC fixture with
  claxon, negotiates the PCM bit depth from the client's client/hello, and
  streams via the real Group push path. It paces pushes so the connection's
  audio backlog stays shallow, and reports the canonical source PCM hash.
• build.py: builds the sendspin-rs-server cargo target.
• implementations.py: flips the sendspin-rs server role to supported,
  scoped to the player/PCM scenarios (the crate's server is player-role,
  PCM v1 — no FLAC/OPUS/metadata/controller/artwork/request-format, so those
  rows stay filtered).

Covered scenarios: client-initiated-pcm (bind + accept),
server-initiated-pcm (dial in), server-initiated-pcm-24bit (24-bit
negotiation).

Validation

Run locally against the sendspin-rs client adapter:

passed: client-initiated-pcm       :: sendspin-rs -> sendspin-rs
passed: server-initiated-pcm       :: sendspin-rs -> sendspin-rs
passed: server-initiated-pcm-24bit :: sendspin-rs -> sendspin-rs

Draft — why

• It points its path dependency at a local repos/sendspin-rs. The
  server role it depends on is not yet in sendspin-rs main — it lives in
  the open server-role PR (Add a server role: synchronized multi-client audio push, mDNS discovery/advertising, and supervised connections sendspin-rs#87). This adapter should land
  after (or alongside) that.
• Against aiosendspin HEAD as the client, the handshake currently fails
  because that client opens with a client/init message (the in-progress
  encryption negotiation); the sendspin-rs v1 server expects client/hello
  first. Flagging for awareness — not worked around here.

Marked draft until the upstream server role lands and the cross-implementation
runs are confirmed on a host that prebuilds all adapters.

[davidgraeff]

TODO: How should I reference the sendspin-rs