feat(A.4 #29): plain-text pre-formatter before LLM cleanup

New crates/ai-formatting/src/to_plain_text.rs module with one public
function: to_plain_text(&[Segment]) -> String.

Rules the function enforces:
- each segment's text is whitespace-normalised (any run of unicode
  whitespace collapses to a single ASCII space, so tabs, newlines,
  and NBSPs never reach the LLM),
- empty and whitespace-only segments are dropped,
- remaining segments are joined with a single ASCII space,
- the joined string is normalised again (so a segment ending in a
  space followed by one starting in a space does not produce a double
  space) and trimmed end-to-end.

pipeline.rs's inline join is replaced with this call. Whisper's
timestamp fields (Segment.start / .end) are carried separately and
never reach the LLM by construction — the "timestamps stripped"
half of brief item #29's acceptance falls out of using Segment.text
alone. The work the module actually adds is whitespace discipline
and the tested boundary (empty input, empty-only input, NBSPs,
pathological whitespace runs, idempotence, double-space at join
boundaries).

Source: Scriberr PR #288 — feeding raw Whisper JSON (with timestamps
and per-segment structure) degraded cleanup quality; plain-text
input raised it back.

crates/ai-formatting/src/pipeline.rs

@@ -2,7 +2,7 @@ use kon_core::constants::SMART_PARAGRAPH_GAP_SECS;
 use kon_core::types::Segment;
 use kon_llm::LlmEngine;
 
-use crate::{llm_client, rule_based};
+use crate::{llm_client, rule_based, to_plain_text::to_plain_text};
 
 /// Post-processing options for a transcription pipeline run.
 pub struct PostProcessOptions {
@@ -68,12 +68,12 @@ pub fn post_process_segments(
 
     if let Some(engine) = llm {
         if engine.is_loaded() && options.format_mode != FormatMode::Raw {
-            let joined = segments
-                .iter()
-                .map(|segment| segment.text.trim())
-                .filter(|segment| !segment.is_empty())
-                .collect::<Vec<_>>()
-                .join(" ");
+            // Plain-text pre-formatter (brief item #29): collapse
+            // segments into a single natural-language string before
+            // the LLM call. Whitespace normalisation + empty-filter
+            // live in `to_plain_text`; the pipeline's job here is
+            // deciding whether to invoke the LLM at all.
+            let joined = to_plain_text(segments);
 
             if !joined.is_empty() {
                 // Pipeline-internal cleanup (used by file-based + live