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  (A.2 #13) has since superseded SpeechToText, and the Registry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/ai-formatting/src/llm_client.rs

@@ -23,7 +23,6 @@ use kon_llm::{EngineError, LlmEngine};
 ///
 /// Both are regression-tested below; neither should be dropped in a
 /// refactor without explicit discussion.
-#[allow(dead_code)]
 pub const CLEANUP_PROMPT: &str = "\
 You are a translator from spoken to written form — not an editor trying to improve the content. \
 The text you receive is TRANSCRIBED SPEECH from a voice recording. \
@@ -56,7 +55,6 @@ Output rules: \
 /// correct them in context without changing the core prompt.
 ///
 /// Returns an empty string if terms is empty.
-#[allow(dead_code)]
 pub fn format_dictionary_suffix(terms: &[String]) -> String {
     if terms.is_empty() {
         return String::new();