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  (A.2 #13) has since superseded SpeechToText, and the Registry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).

crates/core/src/lib.rs

@@ -3,12 +3,11 @@ pub mod error;
 pub mod hardware;
 pub mod model_registry;
 pub mod process_watch;
-pub mod providers;
 pub mod recommendation;
 pub mod types;
 
 pub use error::{KonError, Result};
 pub use types::{
     AudioSamples, DownloadProgress, EngineName, Megabytes, ModelId, Segment, Transcript,
-    TranscriptMetadata, TranscriptionOptions,
+    TranscriptionOptions,
 };