feat(kon): add ai-formatting crate — filler removal, British English, text pipeline

- Filler word removal using word-boundary regex (fixed regex-lite lookbehind limitation)
- British English conversion: 26 -ize/-ise patterns, -or/-our, -er/-re, -ense/-ence
- Text formatting: sentence capitalisation, spacing cleanup
- Hallucination filter: blank_audio, music, silence, auto-thanks detection
- Post-processing pipeline: composed from pure functions, supports Raw/Clean/Smart modes
- Smart mode inserts paragraph breaks on >2s pauses between segments
- 12 tests passing, clippy clean

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ai-formatting/src/llm_client.rs

@@ -0,0 +1,2 @@
+// Stub for future LLM sidecar integration.
+// Will implement TextProcessor trait when mistral.rs sidecar is added.