fix: restore the line continuations of 28 log and error strings

[RobbinBouwmeester]

28 string literals in log, warning, error and assertion messages carried a run of 14 to 26 spaces in the middle of a sentence. For example, the rescore warning read:

rescore: too few decoys to train a classifier; ranking by the untrained              prelim_score with plain target-decoy competition

Each was a \ line continuation whose backslash and newline had been removed, leaving the space before it and the next line's indentation. This PR splits each run back into \ + newline + indentation. That restores the original source, and the message again has the single space it meant.

• 16 files: fdr.rs, index.rs, main.rs, sidecar.rs and the compete, convert, extract, features, quant, rescore, rt_im_train, run, run_experiment, search_seed and seed_pool stages, plus 5 assertion messages in tests/pipeline.rs.
• Deliberate alignment is unchanged: the \x20 padding in mumdia doctor output and the benchmark tables in tests/bench_fragindex.rs.
• run.rs shows a large diff, but it is whitespace only (git diff -w has 5 changed lines). With the over-long string gone, rustfmt can format the enclosing let ... = if cfg.groups.window_groups > 1 { expression it had skipped.
• No behaviour changes. A scan for the same pattern finds none left in the Rust sources and none in scripts/.

Checked: cargo fmt --check, clippy -D warnings, workspace tests.

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