fix(tui): show real file line numbers in Edit tool diffs (#870)

[edenreich]

Summary

The Edit tool rendered its diff from the old_string→new_string snippet, so diffview numbered the changed lines 1, 2, 3… instead of their actual positions in the file (issue #870).

Root cause: the snippet has no knowledge of where it sits in the file, and after the edit runs the file no longer contains old_string, so the renderer can't re-derive positions. The approval preview already diffs whole-file content (correct numbers), but the completed result card and the pending approval card render the snippet.

Fix

Thread the edit's start line through as a gutter offset:

• diffview: new LineNumberOffset(n) shifts hunk FromLine/ToLine once in computeDiff, so digit width, hunk header, and both unified/split layouts all pick it up.
• DiffRenderer.SetStartLine(n) applies LineNumberOffset(n-1).
• EditToolResult.StartLine is recorded in executeEdit (line of the first replaced occurrence) and passed by FormatForLLM for the completed card.
• The pending card computes the start line from the still-unedited file via snippetStartLine.

Tests

• TestLineNumberOffsetShiftsGutter (diffview) — gutter and hunk header reflect the offset.
• New FormatResult case asserting the completed Edit diff shows real file line numbers.

Notes

• Not applied to MultiEdit: its per-edit snippets shift as edits apply, so a single offset isn't correct there.
• Internal-only UI fix — no docs ticket needed.