feat: add reviewed bulk cleanup operations

PLAN.md

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+# Bulk cleanup
+
+## Purpose
+
+Provide reviewed bulk transformations for catalogue values while preserving scope visibility and a
+single undo step for each accepted operation.
+
+## Scope
+
+1. Preview selected rows and columns before and after an operation.
+2. Support whitespace cleanup, Unicode normalization, case normalization, delimiter splitting or
+   joining, regex replacement, and duplicate detection/removal.
+   - Use NFC for canonical Unicode normalization.
+   - Offer NFKC as a separate compatibility-normalization operation because it can change meaning.
+   - Use Unicode default casing for lower-, upper-, and title-case operations. Do not use the host
+     locale.
+3. Report skipped and failed cells without silently changing them.
+4. Apply an accepted transformation as one complete undoable batch.
+
+## Non-goals
+
+- Unreviewed automatic cleanup.
+- Spreadsheet formulas or presentation formatting.
+- Replacing the current find-and-replace flow where it remains the simpler operation.
+
+## Dependencies and merge notes
+
+Do not implement the preview/confirmation UI until PR #118 has landed. That PR changes the table
+panel and delegate surfaces this work would otherwise conflict with. The transformation semantics
+and fixtures may be designed before then.
+
+## Definition of done
+
+- Every operation shows the affected-cell preview and scope before it writes.
+- Cancel makes no changes; apply is one undo step.
+- Tests cover filtered rows, selection scope, no-op values, and failures.