refactor(cr-2026-04-22): remove dead code and stale allow(dead_code) suppressions

2026-04-22 review MINORs and NITs:

- crates/core/src/providers.rs: delete entire module. SpeechToText /
  TextProcessor / ProviderRegistry were forward-looking traits that
  never got wired — the Transcriber trait in kon-transcription
  (A.2 #13) has since superseded SpeechToText, and the Registry
  pattern was redundant against LocalEngine. Keeping them as dead
  public surface signalled future direction that is no longer
  accurate.
- crates/core/src/types.rs: delete TranscriptMetadata. Forward-
  looking struct with an unfulfilled TODO; storage has evolved
  independently through v7/v8 migrations without adopting it.
- crates/ai-formatting/src/llm_client.rs: remove #[allow(dead_code)]
  from CLEANUP_PROMPT and format_dictionary_suffix. Both are
  actively called; the suppressions would hide future genuine
  dead-code warnings in this regression-sensitive prompt file.
- src-tauri/src/commands/live.rs: remove #[allow(dead_code)] from
  LiveStatusMessage. Every variant (Warning, Overload, Error,
  Finished) is constructed in the module today.
- README.md: update kon-transcription row from "SpeechToText
  trait" to "Transcriber trait" and mention the new streaming/
  module.

Workspace test gate green (225 lib tests across all crates).

README.md

@@ -164,7 +164,7 @@ kon/
 |---|---|
 | **`kon-core`** | Shared types (`Segment`, `Transcript`, `Megabytes`, `ModelId`), constants, the `Engine` / `SpeedTier` / `AccuracyTier` enums, hardware probe (`sysinfo`-based), model registry (Whisper + Parakeet + Moonshine entries), hardware-aware recommendation scoring, `process_watch` for meeting detection. |
 | **`kon-audio`** | `cpal`-based microphone capture with device hotplug + error forwarding, VAD, `rubato` streaming resampler to 16 kHz mono, `symphonia` file decoding, `hound` WAV I/O. |
-| **`kon-transcription`** | `whisper-rs` backend (`WhisperRsBackend`) that owns a `WhisperContext` and supports `set_initial_prompt`. `LocalEngine` wraps both Whisper and Parakeet (via `transcribe-rs` ONNX) behind a common `SpeechToText` trait. Model manager handles downloads, paths, and disk checks. |
+| **`kon-transcription`** | `whisper-rs` backend (`WhisperRsBackend`) that owns a `WhisperContext` and supports `set_initial_prompt`. `LocalEngine` wraps both Whisper and Parakeet (via `transcribe-rs` ONNX) behind a common `Transcriber` trait. Streaming primitives (`VadChunker`, `LocalAgreement`, buffer trim) live in the `streaming/` module. Model manager handles downloads, paths, and disk checks. |
 | **`kon-llm`** | `llama-cpp-2` engine with Qwen3 model manager. Three high-level surfaces: `cleanup_text` (formatting), `decompose_task` (3–7 micro-steps, GBNF-constrained JSON array), `extract_tasks` (optional-array, GBNF-constrained). Resumable HTTP downloads with SHA-256 verify. |
 | **`kon-ai-formatting`** | Post-processing pipeline: filler removal, British English conversion, anti-hallucination filter, smart paragraph breaks on long pauses, optional LLM cleanup. Also hosts the `llm_client::CLEANUP_PROMPT` constant (prompt-injection-hardened). |
 | **`kon-storage`** | SQLite via `sqlx` 0.8. Migrations, CRUD for transcripts / tasks / subtasks / profiles / profile terms / settings / error log, FTS5 search, file-storage paths. |