Better dry-run and diff preview

[httpdss]

Summary
Dry run and console output are helpful but lack structured diffs and per-file change summaries.

Proposed changes

• Add --diff option to show unified diffs for files that would be created/modified.
• In console output mode, print a per-file action summary (create/update/skip/backup/rename) and optionally the diff.
• Provide a final summary: counts per action and total files affected.

Files

• struct_module/commands/generate.py
• struct_module/file_item.py
• docs/usage.md and docs/quickstart.md (usage examples and screenshots/GIFs)

Acceptance criteria

• Running with --dry-run --diff shows unified diffs for any modified file.
• Console output mode includes per-file action headers.
• Tests cover diff generation and summary output.