Reduce initial playback stutter

[kahrendt]

Two changes give the decode pipeline more slack at stream start:

• Insert extra silence after the first playback notification, before the first decoded chunk reaches the sink, so the decoder has time to stay ahead. The amount is configurable via PlayerRoleConfig::extra_startup_silence_ms (default 50 ms; 0 disables).
• On encoded ring-buffer underflow, feed silence to keep the DAC fed instead of letting it run dry. Gated on aligning so it only runs during startup/post-seek alignment, not in steady state where stuffing silence at stream end would pile up in the sink and delay a rapid restart.

Adds the frame_aligned_silence_bytes() helper and fill_underflow_silence() to keep the sync task readable, and documents the behavior in the integration guide and internals docs.

Breaking change: the PlayerRoleConfig has been modified with a new field inserted in the middle.

[Copilot]

Pull request overview

Note

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This PR reduces initial playback stutter by adding configurable extra startup silence and by bridging encoded-chunk underflows with short silence writes during alignment (startup/post-seek), keeping the DAC fed until data arrives.

Changes:

• Add extra_startup_silence_ms to PlayerRoleConfig and document it in integration docs.
• Introduce frame-aligned silence sizing and queue/drain extra startup silence after the first playback-progress callback.
• During alignment, feed short “keepalive” silence slices on ring-buffer underflow to avoid DAC underruns.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/sync_task.h	Declares fill_underflow_silence() and documents the new underflow-bridging behavior.
src/sync_task.cpp	Implements frame-aligned silence calculation, extra startup silence draining, and alignment-only underflow silence fill.
include/sendspin/config.h	Adds new public config knob extra_startup_silence_ms with a default.
docs/internals.md	Updates state-machine docs to describe extra startup silence and alignment-only underflow fill.
docs/integration-guide.md	Documents the new configuration option and shows it in example config.

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