[diffs/edit] Fix dedupe line edits and preserve blank lines on indent

[ije]

• Line indent/outdent multi-selection dedupe (3) — indent dispatch
  concatenates per-selection edits with no shared-line dedupe, so a line under
  two carets/ranges indents twice; the outdent variant emits two identical
  deletes that fail overlap validation and the whole command throws.
• Block indent dirties blank lines (1) — a multi-line block indent inserts
  the indent unit on empty and whitespace-only lines inside the selection,
  injecting trailing whitespace on lines the user never touched (outdent
  round-trips it away, bounding the damage).

Deduplicate shared-line edits across multiple selections to prevent double
indentation and overlapping outdent deletes. Skip empty and whitespace-only
lines during block indentation.

• Dedupes line-based indent/outdent edits with a Set, while preserving each selection’s caret remapping.
• Multi-line block indent now skips empty and whitespace-only lines in the existing linear pass.

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