Add WhisperCPP migration plan design doc

[dudarev]

Motivation

• Add a concrete, speed-focused plan to migrate the transcription adapter to WhisperCPP to reduce end-to-end latency (especially time-to-first-token) and enable incremental note rendering while preserving existing architecture and providing a Parakeet fallback.

Description

• Add docs/design/current/2026-01-23-whispercpp-migration-plan.md which outlines discovery and baseline benchmarking, WhisperCPP integration steps (adapter, service, presentation changes), validation and rollout, Parakeet evaluation as a fallback, risks, deliverables, and success criteria.

Testing

• No automated tests were run because this is a documentation-only change.

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Codex Task