Add Slice 4 tuner stack with AGENTS-compliant core types and HTTP API.
Introduces Si4684/ADAU1701 drivers, TunerService, FrequencyKHz, SeekDirection, /api/tuner routes without file-scope globals, host tests, and firmware blob tooling (binaries remain gitignored). Co-authored-by: Cursor <cursoragent@cursor.com>
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