output.harbor: refactor listener management, add dedicated encoder mode

[toots]

Reason

output.harbor was overdue for a cleanup. The old implementation used a Duppy monad-based async state machine with condition variables to manage connected clients, which was more complex than necessary and harder to reason about. This PR also addresses several scalability issues.

Changes

Architecture

Replaces the client state machine with a simpler synchronous listener record. Writes are moved off the streaming thread entirely into a single Duppy task that waits on listener socket write events (\Write fd`), fires only when at least one socket is ready, writes to all listeners non-blocking, and re-registers itself. This decouples the streaming cycle from network write latency.

New dedicated_encoder mode

A dedicated_encoder boolean parameter (default false) selects between two output classes:

• shared_output (default): one encoder shared across all listeners, each seeded with the codec header and burst data on connect
• dedicated_output: a fresh encoder created per listener at connect time, ensuring a clean stream from the first byte — particularly useful for %ffmpeg in copy mode

Scalability fixes

• Write loop off streaming thread: writes now happen in a Duppy task, so slow listeners never stall the streaming cycle
• No buffer copy in write path: per-listener buffers use Strings.Mutable (thread-safe); writes iterate chunks directly via Bytes.unsafe_of_string with no allocation
• ICY metadata bug fix: metadata was previously consumed by the first listener in the list per frame, leaving all others stale; it is now captured once and shared across all listeners

Breaking API changes

• user and password parameters removed; authentication is now exclusively via the auth function, which now receives a record {address, login, password} instead of individual arguments
• burst parameter is now nullable (null disables burst)

Other improvements

• ICY metadata constants renamed (max_title → max_icy_title, etc.) and formatting extracted into standalone functions
• Better variable naming throughout (buflen → buffer_limit, metaint → metadata_interval, etc.)
• Shared logic moved into a virtual base class

Documentation

• doc/content/harbor.md: new output.harbor section documenting dedicated_encoder, auth, and listener callback signatures
• doc/content/migrating.md: breaking changes documented under "From 2.4.x to 2.5.x"