refactor(encode): Phase 2 — promote encoders to pkg/audio/encode (gate met)

[chrisuthe]

Phase 2 of the three-repo split (2 of 3 promotions) — and it completes the Phase 2 gate.

Change

Promote the Opus/FLAC encoders from internal/server to a public pkg/audio/encode, and repoint the three pkg/sendspin users.

• opus_encoder.go / flac_encoder.go (+ tests) moved verbatim (git mv, history preserved), package server → package encode, plus a doc.go (matching pkg/audio/decode).
• role_player.go, server.go, server_client.go: import pkg/audio/encode, reference encode.{Opus,FLAC}Encoder / encode.New{Opus,FLAC}Encoder.

Why this finishes the gate

The encoders turned out to be the only thing pkg/sendspin used from internal/server (the source decoders and TUI are used solely by cmd/sendspin-server). So this PR drops the SDK's internal/server dependency entirely:

go list -deps ./pkg/...  →  ZERO sendspin-go/internal/ packages

Phase 2 gate ("SDK has zero cross-boundary internal/ imports") is met after discovery (#141) + this PR. internal/server (sources + TUI) is now used only by cmd, so it can stay private to the future server-CLI repo.

Note: third promotion downgraded to optional

The planned pkg/audio/source promotion is no longer required for the boundary — only cmd imports the sources, not the SDK. Whether to promote them becomes a product decision (does the SDK ship built-in file/MP3/FLAC/HLS sources for library consumers, or are sources a CLI concern?). Captured in the plan; decide before/with Phase 3.

Verification

• go build ./..., full go test -race ./... (10 packages) — green. gofmt + lint clean. go doc ./pkg/audio/encode renders.
• CGO/opus note: the encoders use opus.Encoder (not opus.Stream), so the -tags=nolibopusfile behavior is unchanged by the move — same as when they lived in internal/server.