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/lib.rs

@@ -2,8 +2,10 @@ pub mod correction_learning;
 mod llm_client;
 pub mod pipeline;
 pub mod rule_based;
+pub mod to_plain_text;
 
 pub use correction_learning::extract_corrections;
 pub use llm_client::{cleanup_text as llm_cleanup_text, LlmPromptPreset};
 pub use pipeline::{post_process_segments, FormatMode, PostProcessOptions};
 pub use rule_based::{format_text, is_hallucination, remove_fillers, to_british_english};
+pub use to_plain_text::to_plain_text;