fix(kon): normalise British English table, single-pass whitespace collapse, byte-index safety

- Normalise BRITISH_REPLACEMENTS: remove baked-in \b from entries so all
  entries are plain base words; the function adds boundaries uniformly
- Replace O(n*m) while-loop double-space removal with single-pass collapse
- Add debug_assert! documenting ASCII assumption for byte-indexed suffix slicing
- Expand llm_client.rs module-level doc comment
- Run cargo fmt on ai-formatting crate

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

crates/ai-formatting/src/llm_client.rs

@@ -1,2 +1,5 @@
-// Stub for future LLM sidecar integration.
-// Will implement TextProcessor trait when mistral.rs sidecar is added.
+//! Placeholder for future LLM sidecar integration (e.g., mistral.rs for smart formatting).
+//!
+//! When implemented, this module will expose a client that sends transcription
+//! segments to a local LLM for context-aware punctuation, paragraph splitting,
+//! and stylistic cleanup beyond what the rule-based pipeline can achieve.